PRIME IDEALS IN RINGS OF CONTINUOUS FUNCTIONS
By
CHAWNE MONIQUE KIMBER
'^ ^ '^ : '1' ..A
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1999
This work is dedicated to those women who preceded me and to those who
are yet to follow.
• ^r;;>
ACKNOWLEDGMENTS
First and foremost, I express my wholehearted gratitude to my advisor, Jorge
Martinez. In the past few years, his humane guidance has helped me to achieve so
very much, in fact, more than I would ever have hoped. I follow his example both in
becoming a mathematician and a caring teacher, and in the enjoyment of the finer
things in life like wine, cheese, and chocolate.
Also, sincere thanks go to my committee members: Richard Crew, for show
ing me some algebra; Alexander Dranishnikov, for teaching me a heap of topology;
Scott McCuUough, for introducing me to real analysis (back when we were both
much younger); and Mildred HillLubin, for expanding my worldview through lit
erature. . "
Cheers and warm hugs to my friends, neighbors, and family, especially to the
immediate: Johnnie, Charles, Prances, Maribell, Chinene, Jean, and the inimitable
Poopygirl, Cei.
TABLE OF CONTENTS
ACKNOWLEDGMENTS iii
ABSTRACT v
CHAPTERS
1 PRELIMINARIES 1
1.1 History 1
1.2 LatticeOrdered Groups 3
1.3 /Rings 7
, 1.4 Rings of Continuous Functions 10
1.5 Approaches 15
2 CHARACTERS 18
2.1 Hahn Groups 18
2.2 Lex Kernels and Ramification 20
2.3 Rank 26
2.4 Rank via Z*Irreducible Surjections 32
2.5 Prime Character 36
2.6 Filet Character 43
3 GENERALIZED SEMIGROUP RINGS 46
3.1 Specially Multiplicative /Rings 46
3.2 rSystems and ^Systems 52
3.3 /Systems 58
3.4 Survaluation Ring and n*''Root Closed Conditions 66
4 RAMIFIED PRIME IDEALS 75
4.1 Ramified Points 75
4.2 Ramified G^points 79
4.3 Ramification via CEmbedded Subspaces 84
5 nvQUASINORMAL /RINGS 88
5.1 Definitions 88
5.2 {B, m)Boundary Conditions 94
5.3 J3X, mQuasinormal and SV Conditions 107
REFERENCES HO
BIOGRAPHICAL SKETCH 113
IV
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
PRIME IDEALS IN RINGS OF CONTINUOUS FUNCTIONS
Chawne Monique Kimber
May 1999
Chairman: Jorge Martinez
Major Department: Mathematics
Given a completely regular topological space X, we wish to determine the
order structure of < Spec(C(X)), C>, the root system of prime ideals of the ring of
realvalued continuous functions on X; and vice versa.
We present four approaches which give partial solutions to these problems.
First, we define three measures on < Spec+(G), C>, the set of prime subgroups of a
latticeordered group, which determine some arithmetic properties of the group, and
vice versa. Second, given any root system, we construct a generalized semigroup
ring U which is a commutative semiprime /ring such that < r(7J),C>, its root
system of values, is orderisomorphic to the given root system. Then we characterize
those nonisolated G^points whose corresponding maximal ideal is the sum of the
minimal prime ideals it contains. Finally, we characterize those spaces X for which
C{X) has the property that the sum of any m minimal prime ideals is a maximal
ideal or the entire ring.
CHAPTER 1
PRELIMINARIES
The focus of this dissertation is the order structure of < Spec(C(X)), C>, the
spectrum of prime ideals of the ring C{X) of realvalued continuous functions on a
topological space, X. To start, we informally present the history and give motivation
for the discussion herein. We then review some essentials about latticeordered
groups, /rings, and rings of continuous functions in detail and then formally indicate
the manner in which this thesis proceeds.
1.1 History
Our history begins with the independent research by Cech and Stone in 1937
(see the papers [Ce] and [St]), in which they describe a compactification 13 X of a
topological space X which has the property that every realvalued continuous func
tion on X extends to a continuous function on PX. Further, via PX, they establish
correspondences between the topological structure of X and certain algebraic prop
erties of its ring C{X) of realvalued continuous functions under pointwise addition
and multiplication. For instance. Stone shows that the maximal ideals of the sub
ring C*{X) of bounded functions are in onetoone correspondence with points of
PX. The map p i^ M*p = {/ € C*{X) : f{p) = 0} witnesses this correspon
dence and is a homeomorphism of topological spaces when the set of maximal ideals
of the ring is endowed with the hullkernel (Zariski) topology. In particular, this
shows that for compact Hausdorff spaces X, Y, we have that X = Y if and only if
C{X) ^ C{Y). The next significant result came in 1939 when Gelfand and Kolo
mogoroff proved in [GK] that the maximal ideals of C{X) are exactly those of the
2
form MP = {/ e C{X) : p e d^xZ{f)}, where Z{f) = {p e X : f(p) = 0} and
pe/3X.
In the 1950's, Gillman, Henriksen, Jerison and Kohls began a formal in
vestigation of topological/algebraic correspondences of this form. The elementary
techniques and results are recorded in the text Rings of Continuous Functions, [GJ].
Concerning prime ideals, in [GJ, 14.3c] we learn that the prime ideals in C(X), con
taining a given prime ideal, form a chain and in [GJ, 4J], it is shown that the
topology on X is closed under countable intersections (i.e., X is a P space) if and
only if every prime ideal of C{X) is maximal. More generally, in [GJ, 14.25] we find
that X has the property that every bounded continuous function on set of the form
X \ Z{f), for some / e C{X), extends to a continuous function on X (that is, X is
an F space) if find only if every maximal ideal of C{X) contains a unique minimal
prime ideal. Knowing these three facts, we can describe the graph of the prime ideal
spectrum of C{X) in each case, where vertices are prime ideals and edges indicate
setinclusion. In the Fspace situation, the graph is a disjoint set of strands (one
for each point of ^X) with no branching; a Pspace yields a graph consisting solely
of vertices (one for each point of PX).
It is from these topological characterizations of the graphical structure of
the spectrum of prime ideals of C{X) that we formulate our questions. Roughly
speaking, we wish to know:
• Is it possible to determine the order structure (under inclusion) of the prime
ideal spectrum of the ring of continuous functions of a given topological space?
• Conversely, given a graph, is it possible to construct a topological space such
that the given graph, in some sense, determines the structure of the spectrum
of its ring of continuous functions?
It turns out that both of these axe rather ambitious pursuits and the ques
tions must be refined before we can approach them. Extending the knowledge of
properties of the prime ideals of C(X), Kohls published a series of papers ([Kl],
[K2], and [K3]) in 1957. In [K2], he addresses the properties of chains of prime
ideals o{C{X). First he shows that the quotient ring C{X)/P is totally ordered for
every prime ideal P and concludes that the prime ideals of the quotient ring form a
chain. Second, it is demonstrated that if P is nonmaximal, then the chain of prime
ideals in C{X)/P contains an 771 sei (that is, a totally ordered set E such that for
every pair of disjoint countable subsets A,B c E such that A < B, there exists
ce E such that a < c and c < 6 for every aE A and every b e B). Hence, the chain
of prime ideals contains at least 2^^ primes. We may thus immediately reduce the
second of our questions to only consider those graphs for which each nontrivial edge
passes through an 771 set of vertices. The facts presented in the next three sections
show us that the class of graphs to consider can be further reduced.
In this dissertation, we continue to refine the questions and present four
perspectivesranging from the very general to the very specificwhich give partial
results. In order to properly introduce these approaches, we must recall some facts
and constructs which are fundamental to the ensuing investigation.
1.2 LatticeOrdered Groups
Let {L, <) be a partially ordered set. If a, 6 6 L are incomparable, then we
write a II 6. L is totally ordered if any two elements are comparable. We say that
L is a lattice if any two elements a,b e L have a least upper bound and a greatest
lower bound, denoted a V 6 and a Ah, respectively. A lattice L is distributive if
a A (6 V c) = (a V 6) A (a V c), and dually for all a,b,ceL.
A group {G, +, 0, <) with partial order < is a lattice ordered group (hence
forth, igroup) if it is a lattice and '\ig<h implies that c\g < c+h and g+c < h+c
for all c E G. The majority of the groups we consider are abelian, so the additive
notation here is for convenience. It is important to note that any ^group is torsion
free [D, 3.5] and its lattice is distributive [D, 3.17]. A (real) vector lattice is an
^group G which is also an Rvector space such that rg > for all positive g € G
and for all positive r e M.
By G"*" we mean the set of elements g eG such that g >0. Each element of G
may be written as a difference of elements of G~^ :let g'^ = gWO and g~ = (g) V 0,
then g = g^  g~. This follows from the fact that g+ A g~ = 0. The absolute value
of an element is given by ^ = g^ + g. In general, we say that a pair of elements
g,hEG axe disjoint ii g Ah = O.We will write g <^ hit ng < h for all n e N.
An ihomomorphism is a group homomorphism that also preserves the lattice
structure. An isuhgroup if of an ^group G is a subgroup which is also a sublattice
of (G, <). We call an ^subgroup convex i{Q<g<h€H implies that g e H. G{S)
denotes the convex ^subgroup of G generated by the set S C G. When S = {g},
we write G{g). In fact, G{g) = {/i G G : 3n 6 N, \h\ < n\g\}. In the special
case that G(g) = G, we call g a strong (order) unit. Let ^{G) denote the set of
all convex ^subgroups of G ordered by inclusion. This set is a distributive lattice
under the operations of arbitrary intersection and Vie/^» = ^(Uie/^«)' where
{Hi}ieT Q ^(G) and / is any indexing set; see [D, 7.10] for details.
Let S CG. Then the polar of 5" is given by
S^ = {g€G: \g\ A s = for all s e S}.
If 5 is a singleton, say S = {g}, then we write g^ for the polar of S. Such a polar
is called principal. If p^ = 0, then g is termed a weak (order) unit. Note that for
any 5 C G, we have that 5^ e (t(G) and (S^)^^ = S^. Let <P(G) represent the
set of polar subgroups of G ordered under setinclusion. Then <P(G) C (t{G), but
in general this is not as a sublattice. Under the operations of arbitrary intersection.
±, and Vie/ ^» ~ (U.e/ ^«)"'""'') where {Hiji^j C ^(G) and / is any indexing set,
we have that ^{G) is a Boolean lattice, by [D, 13.7].
The convex ^subgroups of greatest interest to us are the prime subgroups of
G. These are the subgroups H e C(G) for which any one of the following equivalent
conditions is satisfied (see [D, 9.1], [AF, 1.2.10], or [BKW, 2.4.1]):
1. VgAh = then g e H oi h e H.
2. U g,h>0 and gAheH then g e H or h e H.
3. The right cosets of H are totally ordered.
4. The convex ^subgroups of G containing H form a chain.
As suggested by the terminology and the second condition listed above, the
concept of a prime subgroup is related to that of a prime ideal in a ring. The
difference becomes apparent in considering the final equivalent condition which in
dicates that the prime subgroups form a root system. That is, the graph of the
prime subgroups of G, in which nodes indicate prime subgroups and edges repre
sent containment going up, has the property that incomparable elements have no
common lower bound. Illustrating that prime ideals differ firom prime subgroups,
we note that the zero subgroup is the only prime subgroup of the totally ordered
group of integers, Z; whereas, the zero ideal and the ideals which are generated by
a prime integer comprise the set of prime ideals of Z. The structure of the graph of
the prime subgroups is the subject of our investigation. Note that we use Spec+(G)
to denote the set, or spectrum, of all prime subgroups of G and to stand for the
associated graph.
By Zom's Lemma, minimal prime subgroups exist. Let Min+(G) denote the
set of all minimal prime subgroups of G.If P e Min+(G) then by [AF, 1.2.11] we
6
have P = U{g^ : 9 i P} This implies 0{Q) ^ n{P G Min+(G) : F C Q} is the
set \J{g^ giQ}, for a prime subgroup Q C G, by [BKW, 3.4.12].
A basis for an ^group G is a maximal pairwise disjoint set {gC^i^i C G"*" such
that for each i e /, the set {g G G"*" ■ g < gi) is totally ordered. The following is
Conrad's Finite Basis Theorem presented as [D, 46.12] and [C, 2.47]. It will figure
in our discussion in the next chapter.
Theorem 1.2.1. Let G he an igroup. The following are equivalent:
1. G has a finite basis.
2. Min+(G) is finite.
3. <P(G) is finite.
4 There is a finite upper bound on the number of pairwise disjoint elements of
G,
5. There is a finite upper bound on the number of elements of strictly increasing
chains of proper polars.
Another application of Zorn's Lemma establishes the existence of convex i
subgroups which are maximal with respect to not containing a fixed element g €G.
Any such subgroup is generally termed a regular subgroup and specifically called
a value of g. The set of all regular subgroups of G is usually represented by r(G).
Regular subgroups are prime, by [D, 10.4], and a prime subgroup is precisely a convex
^subgroup which is an intersection of a chain of regular subgroups, [D, 10.8]. In
particular, the minimal prime subgroups of G correspond to the maximal chains
in r(G). For these reasons, we call the root system given by r(G) the skeleton of
Spec+(G). By convention, we view r(G) as a partiallyordered indexing set F whose
elements are denoted by lower case Greek letters and then represent the regular
subgroups by Vy for 7 6 r.
Topologize Spec+ (G) using the hullkernel (or Zariski) topology whose open
base is given by U{g) = {P 6 Spec^.(G) : ^ ^ P} for all 5 6 G. In this topology,
Spec+(G) is Hausdorff if and only if Spec4.(G)=Min+(G) by [CM, 1.4]; on the other
hand, in the subspace topology, Min+(G) is always Hausdorff and U{g) n Min+(G)
is both open and closed for every g €G. The space Spec^. (G) is compact if and only
if G has a strong unit, by [CM, 1.3]; it is demonstrated in [CM, 2.2] that Min+(G)
is compact if and only if G is complemented, that is, if and only if for every g €G'^
there is an /i 6 G"*" such that g Ah = and gV his a. weak unit.
1.3 /Rings
Let {R, +,,<) be a ring whose underlying group is an ^group and satisfies
the relations re < sc and or < cs whenever r < s and c > 0. Such a ring is a lattice
ordered ring (abbreviated iring). If an ^ring R also satisfies caA6 = ac A6 =
whenever a A 6 = and c > 0, then R is called an fring. The following is found in
[BKW, 9.1.2]:
Theorem 1.3.1. Let R be an iring, then the following are equivalent:
1. R is an fring.
2. Every polar in R is an ideal.
3. Every minimal prime subgroup of R is an ideal.
It is not difficult to verify that every ^ring which is ^isomorphic to a subdi
rect product of totallyordered rings (with coordinatewise operations) is an /ring.
In [BP], it is shown that the converse of this statement holds when we assume the
Axiom of Choice (abbreviated, AC). Since we routinely apply AC, let us formally
8
state that we will work within the axioms of ZFC. Then we may use this equiv
alent definition of an /ring R in order to obtain this list of arithmetic properties
given in [BKW, 9.1.10], for a,b,ceR:
1. If c> then c(o V b) = caW cb and (a V b)c = acybc.
2. If c > then c{a Ab) = caAcb and (a A b)c = acAbc.
3. a6 = \ab\.
4. If a A 6 = then ab = 0. '
5. a^ > 0.
An Itideal of an ^ring i? is an ideal which is a convex ^subgroup of R. We
call an ^ideal a prime iideal'ii it is also a prime ideal. Let Spec(i?) denote the space
of all prime ideals of R in the hullkernel toplogy. Let Max(i2) and Min(iZ) denote
the subspaces of maximal and minimal prime ideals, respectively. By property (4)
above, we see that prime ^ideals of an /ring are prime subgroups, hence, as in the
case of ^groups, the subset Spec^(i2) of prime ^ideals forms a root system. Denote
the subspaces of maximal and minimal prime ^ideals by Max/(i2) and Min/(i2),
respectively.
We call a commutative ring semiprime if it contains no nonzero nilpotent
elements. In the case of commutative /rings, we have [BKW, 9.3.1]:
Theorem 1.3.2. Let R be a commutative fring, then the following are equivalent:
1. R is semiprime.
2. For any a,be R, we have that \a\ A 6 = if and only if ab = 0.
3. Every polar of R is an (.ideal which is an intersection of prime ideals.
9
I min{R)=Mm{R).
5. R is iisomorphic to a subdirect product of totallyordered integral domains.
We say that an ^ring R with multiplicative identity, 1, has the bounded
inversion property if a > 1 implies that a is a multiplicative unit. By [HIJo, 1.1],
a commutative /ring R with 1 has the bounded inversion property if and only if
Maxi{R) = Max{R).
Let A be any commutative ring. Then, in the hullkernel topology, Min(A) is
a Hausdorff space with a base of clopen sets. U Aisa. semiprime ring, then Spec(.(4)
is Hausdorff if and only if Min(>l) =Max(^); this occurs if and only if A is von
Neumann regular (br absolutely flat,) i.e., for every a E A there exists b £ A such
that a = a^6), see [AM, p. 35]. In [HJ] it is demonstrated that if A is a semiprime
/ring, then Min(>l) is compact if and only if A is complemented (i.e., for every
a€ A there exists b € A such that ab = and a + 6 is not a zerodivisor). Max{A)
is compact for any commutative ring A with identity, and if A is a commutative
/ring with identity which has the bounded inversion property, then the subspace
is Hausdorff, see [HJo, 2.3].
Let >l be a commutative ring with identity and P € Spec(>l). Define
O{P) = {a€A:3b^P,ab = 0}.
If A is also a semiprime /ring, then this is the same as the ^subgroup 0{P) defined
in the previous section. Recall that the localization of AaX P is the subring, Ap, of
the classical ring of quotients of A/0{P) consisting of the elements whose denomi
nator is not in P/0{P). (For a review of this construction and general facts about
localizations, see [AM] or [G]). It is the case that Ap is a local ring whose unique
maximal ideal is generated by P/0{P) and there is a onetoone correspondence
between prime ideals of Ap and the prime ideals Q of ^ such that 0{P) C Q C P.
10
Thus, if A is also an /ring with bounded inversion, then by the root system struc
ture of Spec<(>l), we have that Am = A/0{M) since the quotient ring is already
local with unique maximal ideal M/0{M).
1.4 Rings of Continuous Functions
Let X be a Hausdorff topological space. X is called completely regular (ox
Tychonoff) if for every closed set AC X and x € X\A, there exists a realvalued
continuous function on X such that f{x) = 1 and f{A) = {0}. Unless otherwise
stated, we assume that ail spaces are completely regular. Let C{X) denote
the set of realvalued continuous functions on a space X. Under the operations of
pointwise addition and multiplication, C{X) is a semiprime ring. Order the ring
via: f < 9 a and only if f{x) < g{x) for all x e A". This ordering gives an /ring
structure such that C{X) has the bounded inversion property. Let C*{X) denote
the convex ^subring of bounded functions.
The zeroset of /, is the set Z{f) = {x G A" : /(x) = 0}. The complement,
coz{f) = X \ Z{f), is the cozeroset of /. By [GJ, 3.6], a Hausdorff space X is
completely regular if and only if its topology is the same as the weak topology
generated by C{X). Equivalently, the set of all zerosets, Z{X), is a base for the
closed sets of such a space, [GJ, 3.2].
Sets A,B C X axe completely separated if there exists / 6 C{X) such that
f{A) = {0} and f{B) = {1}. If for every / 6 C{A) there exists / G C{X) such that
/U = /> t^^n we say that A is C embedded in X. Likewise, A is C* embedded in X if
bounded continuous functions on A extend to bounded continuous functions on X.
These embedding properties are characterized by complete separation of particular
subsets, as follows:
1. Urysohn Extension Theorem [G J, 1.17]: A C A" is C"embedded in X if and
only if any two completely separated sets in A are completely separated in X.
11
2. A C"embedded set is Cembedded if and only if it is completely separated
from every zeroset disjoint from it, [GJ, 1.18].
Recall that a HausdorfF topological space X is normal if any two disjoint
closed sets are separated by disjoint open sets. Assuming this stronger separation
axiom, the results listed above give rise to the theorem stated in [GJ, 3D], in which
the equivalence of the first two statements is known as Urysohn's Lemma.
Theorem 1.4.1. Let X be Hausdorff. The following are equivalent:
1. X is normal.
2. Any two disjoint closed sets of X are completely separated.
3. Every closed set of X is C* embedded in X.
4 Every closed set of X is Cembedded in X.
For many reasons, it is often preferable to work with compact spaces. The
StoneCech compactification 0X is our compactification of choice, since PX is char
acterized by the property that it is (up to homeomorphism) the unique compact
space in which X is dense and C*embedded. There are at least three different ways
to construct /SX, we begin with the one based on ultrafilters, described in detail in
Chapter 6 of [GJ], which we now summarize.
Let X be a completely regular space, let C be a subset of the power set of
X and !F CC. .F is a C filter if ^ .F, it is closed under finite intersections and if
for every F 6 .F, the fact that F C F' e C implies that F' € .F. If .F is a Z{X)
filter, then T is also called a zfilter. A maximal filter is an vltrafUter; similarly, a
zultrafilter is a maximal zfilter. Let ^X be the set of all zultrafilters on X which
we index by {A*" : p e px). A closed base for the topology on PX is given by sets
of the form Z={pepX:ZE AP}, for Z 6 Z{X). Let p 6 ySX and define
M" = {/ e C{X) : p € clpxZif)}.
12
The theorem of Gelfand and Kolmogoroff [GK] is stated simply as:
Theorem 1.4.2. For a completely regular space X, the set Max(C(X)) is given by
{MP.pe ^X}.
In fact, this result gives rise to a homeomorphism of /3X with Max(C(X)).
That is, since the sets Z[M^] = {Z{f) : / e M^} are precisely the 2ultrafilters on
X, by [GJ, 2.5], and Theorem 1.4.2 shows that the map p h> Z[M^\ is the desired
correspondence.
If p e X, then we will write Mj, and, in this case, the maximal ideal and
corresponding zultrafilter are called fixed. Otherwise, a maximal ideal and its
corresponding zultrafilter is called free. It is evident that X is compact if and only
if every maximaJ ideal of C{X) is fixed. Maximal ideals are also classified by the
residue field C{X)/M^. Identifying the constant functions with their constant, we
see that these fields always contain a copy of R We call a maximal ideal real if the
field is exactly R; otherwise, the maximal ideal is called hyperreal. This concept is
the basis for considering the Hewitt realcompactification of X. Denoted vX, it is the
smallest subspace of PX in which X is dense and such that every maximal ideal of
C{vX) is real. In fact, by [GJ, 8.5], vX is the largest subspace of pX in which X
is Cembedded.
With these facts about the maximal ideals firmly in place, we now proceed
to consider the nonmaximal prime ideals. We know that every prime ideal of C{X)
is convex, by [GJ, 5.5]; so we deduce that Spec(C(X)) is a root system. In order
to understand this root system, we are required to consider the properties of other
ideals. For instance, for p e pX, the ideals of the form
O^ = 0{M^) = {fe C{X) : dpxZif) is a neighborhood of p}
are of paramount interest when examining the prime ideals of C{X). One reason is
given in [GJ, 7.15]:
13
Theorem 1.4.3. Every prime ideal P in C{X) contains O^ for a unique p e /3X
and AfP is the unique maximal ideal containing P.
If O^ is prime, then we call p an Fpoint. If X has the property that O^ is
a prime ideal for every p ^ X, then we call X an Fspace. We see that in this case,
the graph of Spec(C(X)) consists of a set of strands with no branches. Note [GJ,
14.25]:
Theorem 1.4.4. Let X be completely regular. The following are equivalent:
1. X is an Fspace.
2. pX is an Fspace.
3. The prime ideals contained in any given maximal ideal form a chain.
4 Every cozeroset of X is C* embedded.
5. Any two disjoint cozerosets of X are completely separated.
6. Every ideal of C{X) is convex.
7. Every finitely generated ideal ofC{X) is principal (i.e., C{X) is Bezout^.
A special case of an Fpoint is when Op = M^ and we call p a P point if
this occurs. Call X a Pspace if every point of X is a Fpoint. In this case, the
spectrum of C{X) consists only of vertices. Equivalent definitions of Fspace are
presented in [GJ, 14.29] and are recorded below. First, recall that an ideal / of C(A')
is called a zideal if / e / and Z{f)  Z(g) implies that g e I. It is immediate
from the definitions that M^ and O^ are zideals for all p e pX. Note that not all
prime ideals are jideals; however, the following says that this is the case in a von
Neumann regular ring.
14
Theorem 1.4.5. Let X be completely regular. The follomng are equivalent:
1. X is an P space.
2. vX is an Pspace.
3. Every prime ideal of C{X) is maximal.
4. Every cozeroset of X is C embedded.
5. For each f G C{X), the zeroset Z{f) is open.
6. Every ideal ofC{X) is a zideal.
7. For every f 6 C{X), there exists g G C{X) such that f = gf^ (that is, C{X)
is von Neumann regulaxj.
We now recall the definitions of other types of spaces which are useful to us.
X is basically disconnected if the closure of any cozeroset is clopen. X is extremally
disconnected if any open set has open closure. Discrete spaces are extremally dis
connected; extremally disconnected spaces are basically disconnected and all such
spaces are Fspaces by [GJ, 14N.4]. Every Pspace is basically disconnected by [GJ,
4K.7]. A space is a quasiF space if every dense cozeroset is C*embedded. Clearly,
from [GJ, 14.25], we see that every F space is quasiF. The converses of the pre
ceding statements do not hold. That is, these are distinct classes of spaces, as we
now illustrate.
Example 1.4.6. Consider the following spaces:
1. Let W be a free ultrafilter on N. Let E = N U {a}, in which points of N
are isolated and neighborhoods of a are of the form U U {a}, where U eU.
Then E is an extremally disconnected subspace of ^N, but not a Pspace. In
particular, O,^ is a prime ideal which is not maximal; see [GJ, 4M]. Therefore,
E is an Fspace.
1$
2. Let £) be an uncountable set. Let AD = D U {A}, where points of D are
isolated and a neighborhood of A is given by any cocountable set containing
it. Then AD is basically disconnected, but not extremally disconnected by [GJ,
4N.3]. Moreover, the topological sura X = AD U E is basically disconnected,
but neither extremally disconnected nor a Pspace, by [GJ, 4N.4].
3. The corona, /?N \ N is a quasiF space which is an Fspace, yet not basically
disconnected; see [GJ, 6W.3, 140].
1.5 Approaches :
Starting as generally as possible in Chapter 2, we define three cardinal valued
characters on the spectrum of prime subgroups of an ^group. The value of each
measure determines a portion of the arithmetic and/or polar structure of the £
group, and vice versa. For instance, we define the prime character, 7r(G) of an
^group, G to be the least cardinal k such that for any family {Qa}a<K Q Min+(G),
of distinct minimal prime subgroups, we have that Vq<k Qc, is the smallest convex
^subgroup of G containing all the elements of Min+(G). Roughly speaking, it is
a measure of the complexity of minimal paths in the graph of Spec+(G) between
minimal prime subgroups. We will show that the measure being finite satisfies the
following, where lex(G) denotes the smallest convex ^subgroup of G containing all
the elements of Min+(G) :
Proposition 1.5.1. Let G be an igroup and m a positive integer. The following
are equivalent:
1. 7r(G) =m < oo.
2. m is minimal with respect to the property that lex(G) = G(Uj" ^ aj^) for any
m painvise disjoint positive elements, {aj}^^ C lex(G)+.
16
3. m is minimal such that for any prime P ^ lex((7), the chains of proper polars
in P have length at most m — 1.
Chapter 3 is devoted to a discussion of the properties of F(A, R), the gener
alized semigroup ring of real valued maps on a root system A (which has a partially
defined associative operation, I) each of whose support is the join of finitely many
inversely wellordered sets. The ring structure on this group is introduced in [Cl]
and [C2]; we endow this ring with an /ring structure. In particular, we show that
if (A, I) is a root system such that each of the following holds:
1. + is associative (when it makes sense);
2. if a, /? G A are comparable, then a j /?, /? f a are defined;
3. if a < /? and a I 7, /? + 7 are defined, then a I 7 < /? f 7 and if 7 I a, 7 I /?
are defined then 7 ( a < 7 I /?;
4. and if n is maximal, then 6 + fi,fi + 6,fi + fj, are defined and 6 + fj, = ^ + S = 6
for every (J < /x,
then F(A,R) is an /ring if and only if 6 = a + implies a,l3 > 6. And when
this occurs, the /ring is seraiprime and satisfies the bounded inversion property.
Moreover, by [CHH, 6.1], given any root system A, one of these /rings has A
orderisomorphic to its root system of values. Thus, the second of our questions is
answered in the class of /rings on the level of skeletons.
However, the solution to the second problem remains unclear in the smaller
class of rings of continuous functions. To gain a modicum of clarity on the situation,
we look to the work of Attilio LeDonne, published in 1977 in [Le], in which he
addresses the incidence of branching in the graph of Spec(C(X)). He shows, for
instance, that the root system branches at every prime 2;ideal when X is a metric
space. In [Le, §2], LeDonne includes a result of DeMarco which states that there
17
is branching at each Mp when X is a firstcountable space and p is nonisolated.
In Chapter 4, we show that, for a nonisolated G^point of a completely regular
space, there is branching at Mp if and only if X \p is not C*embedded in X. This
result is then used to examine branching in Spec(C(X)) when X is not necessarily
firstcountable.
Both of our questions are addressed in Chapter 5, in which we generalize a
few of the results of Suzanne Larson on quasinormal f rings that are found in the
series of papers [Lai], [La2], and [La3]. The semiprime commutative quasinormal
/rings with identity are the ones having the property that the graph of the root
system of prime ^ideals does not contain a subgraph of the form:
A
(1.1)
By [La3, 3.5], a normal space X has the property that C{X) is quasinormal if
and only if d{U) PI d{V) is a Pspace for any disjoint cozerosets U,V C X. Our
generalizations similarly describe those normal spaces X for which Spec(C(A')) does
not contain a subgraph of any of the following forms:
k k k k
(1.2)
where n,k,ai,...an are positive integers satisfying some specified conditions.
CHAPTER 2
CHARACTERS
We seek a collection of measures on root systems whose values will determine some
portion of the structure of a latticeordered group. In this chapter we describe three
such measures: rank, prime character, and filet character. The rank measures the
width of a connected component of the spectrum, the prime character determines,
roughly speaking, the complexity of minimal paths between minimal primes, and
the filet character counts the maximum length of a chain of branching incidences.
The first sections of this chapter are a review of two constructs essential to the
discussion to follow.
2.1 Hahn Groups
To begin, we recall a method of constructing examples of ^groups having a
specified root system as the skeleton of its prime spectrum. Let A be a root system
and define
y (A, R) = {u : A ^ R : supp(w) has ACC},
where supp(t;) = {5 G A : v{S) 7^ 0}. V{A, R) is an ^group under pointwise addition
ordered by the relation: u > if and only if v{6) > for every maximal element
6 € supp(i;). This ^group is called a Hahn group. In the paper of Conrad, Harvey
and Holland [CHH], it is demonstrated that any abelian ^group can be embedded
in a Hahn group of a more general description than we give here. Of interest to
us is the ^subgroup of maps with finite support denoted by E(A,R) and the £
subgroup of maps whose support is the join of finitely many inversely wellordered
sets, denoted by F(A,R). Clearly, E(A,R) C F(A,R).
18
19
The proof of the first statement of Proposition 2.1.1 is analogous to that
of Theorem 6.1 in [CHH]. This establishes that r(E(A,R)),r(F(A,R)) and A are
isomorphic as partiallyordered sets. For the sake of completeness, we present an
elementary proof of this fact for the case of F(A,R), although the result is easily
obtained from the theory of finite valued ^groups. The proof is identical in the case
ofE(A,R).
Recall that an ^group is finitevalued if each element has only a finite number
of values. A special value is a prime subgroup which is the unique value of an element.
An ^group G is finitevalued if and only if every value of G is special and if and
only if every element of G is a finite sum of pairwise disjoint special elements; for
details, see [AF, 10.10]. If G has a set S of special values such that 5 is a filter and
OS = {0}, then G is called specialvalued.
Proposition 2.1.1. Let A be a root system. For each 5 € A define
Vs = {f G F(A, R) : t;(7) = when 7 > 6}.
Each Vg is a special value. Further, every value of an element of F{A, R) is of
the form Vs for some 6. Thus, A is the skeleton o/Spec^(F(A,R)) and F(A,R) is
finitevalued. '
Proof: Let 6 G A and let xs ^ F{A, R) be the characteristic function on {5}. Then
Xs ^ Vg and we will show that Vs is the unique value of xs Let V be a value of xs
and let v G V+ \ V^. Then there exists j >S such that 7 is maximal in supp(u), and,
hence, t;(7) > 0. If (J < 7, then < xe < v, a. contradiction. If (J = 7, then there
exists a positive integer n such that < x* < ^^ and hence x,? € F by convexity,
which is a contradiction. Thus V = Vs. ' ^.
Let u € F(A, R)"*" and let V be a value of v. Let D be the finite set of
maximal elements of supp(t;). Then the characteristic function xd is not in V; else.
20
there exists an integer n such that <v < nxo, a contradiction. Since V is prime
and the set {xs ■ 5 E D] is pairwise disjoint, there exists a unique element 5 € D
such that xs t ^ By the above, we know that V C Vs. Finally, since v ^ Vs, we
have that V = V^, as desired.
The final statement follows from [AF, 10.10] since we have shown that every
value is special. ■
2.2 Lex Kernels and Ramification
Throughout, we will describe the location of a prime subgroup in the graph
in reference to a designated convex ^subgroup, called the lex kernel of an ^group G
and denoted by lex(G). It is the least convex ^subgroup containing all the minimal
prime subgroups of G. It is always the case that lex(G) is a prime subgroup [D,
27.2] which is normal in G [D, 27.13]. The following is a summary of a part of the
discussion of lex kernels in [D, §27] and gives a description of the ^subgroup in
terms of its generators.
Proposition 2.2.1. Let G be an (.group and let C be a convex isubgroup. The
following are equivalent:
1. C = lex(G).
2. C is the least prime subgroup such that if < g ^ C then g > h for every
heC.
S. C is the convex isubgroup of G generated by {g ^G : g \\0}.
4.C = {0}u{g€G: ^gu92,...9n e G,g \\ g^  52  •.•  g^ \\ 0}.
5. C is the convex (subgroup of G generated by the nonunits of G.
6. C is prime and is the smallest among all convex (subgroups of G which are
comparable with every convex (subgroup of G.
21
7. C is the maximal convex isubgroup of G such that lex(C) = C.
8. C is the supremum of the proper polars ofG in the lattice of convex isubgroups
of a.
It is natural to now introduce a concept which we will discuss in more detail in
Chapter 4. This is a generalization of a concept from [Le]. Let >1 be a commutative
ring with identity and for each a € A, let Max(a) = {M € Max{A) : a € M}. Recall
that an ideal / of >1 is a zideal if a G / and Max(a) = Max(6) imply that 6 6 /.
Definition 2.2.2. Let A be a commutative fring with identity. A prime iideal P
is ramified if it is the sum of the minimal prime ideals that it contains. A maximal
. ■ ; i j> ■
iideal M is totally ramified if every prime zideoi contained in M is ramified. A
completely ramified ring is one in which every prime zideal is ramified.
Graphically, a prime ^ideal P < Ais ramified if and only if it is minimal or
if the root system of prime ^ideals of A branches at P. We begin with the ^group
characterization of ramification. It is the case that a ramified maximal ^ideaJ M
of i4 is the lex kernel of the local /ring A/0{M). In order to discuss a proper lex
kernel in an ^ring, A, we must operate inside a localization. Henceforth, we will
obtain results for local rings and tacitly extend to the general case by referring to
localizations.
The following characterization of ramified maximal ^ideals is immediate from
Proposition 2.2.1.
Corollary 2.2.3. Let Abe a commutative semiprime local fring with identity and
bounded inversion and let M be the maximal ideal. The following are equivalent:
1. M is ramified.
2. M is the convex isubgroup of A generated by {f ^ A: f \\Q}.
22
3. M = {0}U{feA: 3h,h,...fn € AJ \\ h II /2 II ••• II k II 0}.
4. M is the convex isubgroup of A generated by the set
{feA:3g€A,g>0,gAf = 0}.
5. M is the convex ^subgroup generated by the elements of A which are not order
units.
6. M is the smallest among all convex £subgroups of A which are comparable
with every convex i subgroup of A.
7. M is the supremum of the proper polars of A in the lattice of convex isubgroups
of A.
It is wellknown that the lex kernel of an £group is a prime subgroup (see [D,
27.2]). We now show that the lex kernel of a commutative local semiprime /ring
with identity is an ideal and then give conditions which guarantee that the lex kernel
is a prime ideal.
Let ^ be a commutative /ring with identity. Recall that an ideal / < A is
pseudoprime if afc = implies a € / or 6 6 /. An ideal J < A'ls semiprime if a E J
whenever a"^ e^ J. A is squareroot closed if for any <a e A, there exists <b€ A
such that a = Ir^. Let a,b € A, then A is nconvex if whenever < a < 6", there
exists «€ >l such that a = 6u.
Proposition 2.2 A. Let A be a commutative semiprime local fring with identity.
Then \ex{A) is a prime subgroup which is a pseudoprime iideal. If, in addition, A
is squareroot closed, then lex(A) is a semiprime £ideal.
Proof: Let / 6 lex (A). Then there exists 5 > such that f A g = 0. U af =
then af e lex(A); else, af A g — and we conclude again that af 6 lex(i4). Hence
lex(yl) is an ideal. j
23
Let N be the set of nonunits of ^4 and recall that lex(A) = A{N). Let oft = 0.
If a or 6 is 0, then there isThen by convexity, we see that a^,a~ 6 M^) Hence
a e A{N) and we have that the lex kernel is pseudoprime. Since any prime ideal is
semiprime and the lex kernel is the sum of the minimal prime ideals, the lex kernel
is semiprime, if A is also squareroot closed; see [HLMW, 2.12(d)]. ■
Corollary 2.2.5. Let A be a commutative semiprime local fring with identity and
bounded inversion and let M be the maximal iideal. M is ramified if and only if
\ex{A) is a z ideal.
Proof: Since the maximal ^ideal is the only 2ideal of a local /ring, this is
immediate. ■ ., ^ . . .
Corollary 2.2.6. If A is a commutative local 2convex semiprime fring with iden
tity which is squareroot closed, then the lex kernel of A is a prime iideal.
an
■
Proof: By the remark after [La4, 4.2], under these hypotheses, we have that
^ideal is a prime ideal if and only if it is pseudoprime and semiprime
For the remainder of this section, let G be an abelian latticeordered group.
Recall the following for H an ^subgroup of G. H is rigid in G if for every h e H
there is g € G such that /i'' = g^'. It is shown in [CM, 2.3] that if H is rigid in G
then the contraction of minimal prime subgroups of G to minimal prime subgroups
of /f is a homeomorphism of minimal prime spaces. If /f € <t{G), then H is very
large in G if it is not contained in any minimal prime subgroup of G. It is shown in
[CM] that ii H e €{G) then H is very large in G if and only if H is rigid in G. It
turns out that ramification in a rigid subring indicates global ramification and vice
versa. This is a direct consequence of the lex kernel correspondence demonstrated
below.
24
We will also need the following facts (see [BKW, 2.4.7, 2.5.8]):
Proposition 2.2.7. Let H G it{G).
1. The contraction map from the set of prime (.subgroups ofG not containing H
to the set of prime isuhgroups of H is an orderpreserving bijection.
2. IfV is a value ofh€H in G, then V t^VDH is a bijection between the set
of values of h in G and the set of values of h in H. :■
Proposition 2.2.8. Let H < G be a convex isubgroup. Assume that u € H is a
weak unit of H and a weak unit of G. Let V be a value of u in G. Then we have
that VnH = \ex{H) if and only ifV = lex(G).
Proof: Assume that H C P e Min+(G) then « e P and we have that u^° ^ P by
[AF, 1.2.11]. This is a contradiction since u'^^ = G P. Thus H is rigid in G since
it is a convex ^subgroup which is very large in G.
Assume that V n if is the lex kernel of H. Then V H H is the least convex
^subgroup of H containing all the minimal prime subgroups of H. Since H is rigid
in G, by the bijection given in the first part of Proposition 2.2.7, all the minimal
prime subgroups of G are contained in V, and V is the least such convex ^subgroup
of G. That is, if W" ^ y also contains the minimal prime subgroups of G, then
WnH is a. convex ^subgroup of H containing all the minimal prime subgroups of
H and hence WnH = VnH. But this says that W f) H e r{H) is a value of u
and hence, W Er{G) is a. value of u Therefore V = W.
If y is the lex kernel of G then VOH contains all the minimal prime subgroups
of H. Thus the lex kernel of H is contained in V H H. Let P C V (1 H he a,
prime ^subgroup of H containing all the minimal prime subgroups of H. Then by
Proposition 2.2.7, there exists a prime convex ^subgroup Q < G not containing
H such that P = Q Ci H and since we have a rigid embedding, Q contains all the
25
minimal prime ^subgroups of G. Hence, Q = V, P = Vr\H and V n H is the lex
kernel of H. ■
Let A be a commutative semiprime /ring with identity and bounded in
version. If M € Max{A) then Am is semiprime with bounded inversion. This is
a result of the wellknown facts that the ^homomorphic image of an /ring with
bounded inversion has bounded inversion and that Am — A/0{M); see the proof
of [La3, 2.7]. Since we must localize an /ring in order to have a proper lex kernel,
the following allows application of Proposition 2.2.8 to /rings.
Proposition 2.2.9. Let B be a commutative semiprime fring with identity and
let A be a rigid convex fsubring of B. If M G Max/ (A) is such that M = N Ci A
for some N e Max/(B), then Am is a rigid convex fsubring of Bn
Proof: Recall that Am = A/0{M) and Bn ^ B/0{N). Define a map <I>:A^Bn
by a t^ a+0{N). This map is an /ring homomorphism. We show that the image is
convex in Bn and that the kernel is 0{M). For a G A, let < b+0{N) < a+0{N).
Then there exists n E 0(iV)+ such that < 6 < a+n. If bn < then < 6 < n and
hence b G 0{N) since 0(N) is convex in B. Thus we may assume that < b—n < a.
Then bn e Aajidb + 0{N) = bni0{N) elm{(f)). Therefore the image of </) is
convex in Bff.
The kernel of </> is 0{N) n A. It is easy to show that 0{N) OAC 0{M) since
M = N n A. For the reverse inclusion, assume that a € A and a ^ 0{N). Since
0{N) is the intersection of the minimal prime ideals of B contained in N, there exists
P GMin(B) such that P C AT and a ^ P. By the rigidity of A in B, PnA eMm(A),
and therefore a ^ 0{M). We now have that Ker((^) = 0{N) nA = 0{M) and
therefore Am is a convex /subring of Bn.
Since Am contains the identity element of Bn, Am is very large in Bn For
rigidity, we need only recall that very large convex embeddings are rigid, [CM]. ■
26
Corollary 2.2.10. Let B be a commutative semiprime fring with identity. Let A
be a rigid convex fsubring of B. Let M 6 Maxi{A) be such that M = N n A for
some N G Max/(B). Then M is ramified in A if and only if N is ramified in B.
Let A* denote the /subring of bounded elements of the commutative semiprime
/ring A with identity. Note that A* is convex and rigid in A. In his dissertation
[Wo], Woodward proves the following fact:
Theorem 2.2.11. Let A be a semiprime fring with identity and bounded inver
sion. Let M be a mojcimal ideal of A and let M be the unique vaiue of A* containing
MnA*. The map M i¥ M gives a homeomorphism between Max(^) and Max{A*).
That is, Max(A*) is the subspace consisting of values of\ in A*. In particular, if M
is a real maximal ideal of A, then M HA* e Max(^*).
Corollary 2.2.12. Let A be a commutative semiprime fring with identity satisfy
ing the bounded inversion property. Let M e Max (A) be real. Then M is ramified
if and only if M = M CiA* is ramified in A*.
2.3 Rank
The first character on Spec+ (G) that we consider is simply one which counts
the minimal prime subgroups contained in a convex ^subgroup.
Definition 2.3.1. The rank, rkG(H) of a convex isubgroup H < G is the cardi
nality of the set of minimal prime subgroups of G contained in H. If that cardinal
is not finite, then we will say that H has infinite rank; we may choose to specify the
cardinal when its value is of significance in a discussion. If H is a minimal prime
subgroup ofG, then we define rkotH) = 0.
This is a variation of the following definition given in [HLMW]: Let ^ be a
commutative /ring with identity and M a maximal ^ideal of A. The rank of M,
denoted rkyi(M), is the cardinality of the subspace of minimal prime ideals of A
27
contained in M. By convention, if the rank of M is infinite and we don't necessarily
care about the exact cardinality, we write rk^(M) = oo. The rank of a point p E X,
rkxip), is the rank of Mp. The rank of the /ring A is the supremum of the ranks
of the maximal ^ideals of A, when it exists; the rank of a space X is the rank of
c{xy
We begin with illustrations of the extremal values of ranks. An ^group is
semiprojectable if for any ^,/i e G+, (5 A h)^ = G{g^ U b^). In [BKW, 7.5.1], it
is proved that G is semiprojectable if and only if each prime subgroup contains
a unique minimal prime subgroup, which is equivalent to rkcCP) < 1 for every
P € Spec^(G). Thus, it is evident that a space X is an Fspace if and only if C{X)
is semiprojectable which is equivalent to rk(C(A')) < 1. In particular, X is a Pspace
if and only if C{X) is von Neumann regular, which is equivalent to rk(C(X)) = 0.
The onepoint compactification of the natural numbers, aN, is an example of a space
for which C{X) has infinite rank, [GJ, 14G]. In fact, if a is the point at infinity,
then the maximal ideal corresponding to a contains 2* minimal prime ideals  one
for each free ultrafilter on N. Moreover, by [HJ, 4.8], this subspace of minimal prime
ideals is homeomorphic to the corona, ^OT^ \ N. Proposition 2.3.3 describes a general
situation in which we have infinite rank.
We recall some definitions. Prom [LZ, 39.1]: let G be a vector lattice, v e G+,
and let {^n}^i C G be a sequence. We say that the sequence converges relatively
uniformly to g eO along the r^ulator v, and write gn ^ g, if for every e > there
exists iVe > such that for all n>Ne,we have that \ggn\<£v. The sequence is
relatively uniformly Cauchy with respect to v if for every e > there exists Ng >
such that for all n,m > N^, we have that \gm  gn\ < ev. G is called uniformly
complete if for every u € G+, every sequence which is relatively uniformly Cauchy
with respect to v relatively uniformly converges along the regulator v.
28
Lemma 2.3.2. Let G be a uniformly complete vector lattice with weak order unit
u € G"*". For any set {gj}j&w, */iere exists g eG such that g = fljea;^/
Proof: Let5 = E]^i2^(bA«). ■
Proposition 2.3.3. Let G bea uniformly complete complemented vector lattice with
weak order unit u e G'^. If for some Q G Spec+(G) we have TkaiQ) > w, then Q
contains at least 2* minimal prime subgroups. ,,, . , ^
Proof: Note that, by [CM, 2.2], Mm^{G) is compact since G is complemented. Let
V = {P„}neN be a countably infinite set of minimal prime subgroups which are
contained in Q which is discrete in the huUkerael topology on Min+(G). We first
show that P is C*embedded in Min+(G) and conclude that the minimal prime space
contains an homeomorphic copy of ;9N. Then we describe the elements of Min+(G)
that correspond to the points in this copy of /3N \ N.
Let A,B C V he completely separated in V and index them by 7, J C N
as A = {Ai : i € 1} md B = {Bj : j e J}. Fix i G / and let Ai e A. For each
Bj e B, Let Oi. G A+ \ Bj and &j. G Bf \ Ai. Then A< G OjeJ^i^Ji) =^ ^< ^^
B C \J.^j U{ai.) = t/(Ej6j 2'(oi. A u)) =^ Li. Then Ki, Li are disjoint closed sets
in Min4.(G).
For each i G /, generate the disjoint pair ATj, Lj. Then A C d{Ui^iKi) = K
def
and B C Djg/Lj = L. By Urysohn's Lemma, the disjoint closed sets K^L are
completely separated in Min^.(G) since Min+(G) is normal. Consequently, A, B are
completely separated in Min+(G) and therefore P is G*embedded in Min+(G) by
the Urysohn Extension Theorem. Finally, by [GJ, 6.5], the closed subset of Min+(G)
of minimal prime subgroups in Q contains an homeomorphic copy of /?N.
Let W be a free ultrafilter on N. For g £G, let N{g) = {n: g e Pn}. Define
a new prime subgroup F by ^ G F if and only if N{g) G U. We show that F is a
29
minimal prime subgroup. The following proof is the same as that for [HJ, 4.8] and
for [HLMW, 4.1].
Let g,h e P. P is a. subgroup since N{g  h) D N{g) n N{h) 6 U implies
g  h e Phy filter properties. By convexity and since the F^ are prime subgroups,
N{g\/h) D N{g) 6 U and N{gAh) D N{g) €U,vfe have that F is a sublattice of G.
Thus F is an ^subgroup of G. Let < 5 < /i G F. Then N{g) D N{h) G U since each
F„ is convex, and thus F is convex. Let gAh€ P, then N{g)\JN{h) D N{gAh) e U
implies that N{g) or N{h) is in U since U. Thus F is a prime subgroup of G.
Let g E P. Since F„ is a minimal prime subgroup for each n, we have that for
each n G N{g), there exists an /i„ e G \ F„ such that 5 A /i„ = 0. By Lemma 2.3.2,
there exists h e G such that h^ = rinejvffl) ^n Then g Ah = and /i ^ F„ for all
n e Ar(fli) since h^ C h^ C F„ for each n G iV(5). Thus iV(5) n H{g) ^ W and hence
h ^ P and F is a minimal prime subgroup of G. ■
Recall that a space X is cozerocomplemented if for any cozeroset U C. X
there exists a cozeroset V C X such that U C\V = and C/ U V is dense in X.
A concrete example of a maximal ideal of infinite rank is found in C{X) where
X is cozerocomplemented and first countable. DeMarco shows in [Le, §2] that
rk (Mp) > 2, for any nonisolated point of a first countable space (the result actually
says more than this, and we will discuss this in Chapter 4). By modifying DeMarco 's
proof, we show that Mp contains infinitely many minimal prime ideals and hence
has rank at least 2'.
Proposition 2.3.4. Let X he first countable and let p € X he nonisolated. For
every m G N there exists a family of m distinct prime ideals which sum to Mp.
Moreover, if X is also cozerocomplemented, then there exist at least 2* minimal
prime ideals contained in Mp.
30
Proof: Since p has a countable base, there exists g e C{Xy such that Z{g) = {p}.
Let {Vi}J^i be a neighborhood base at p and define a sequence of real numbers {a„}
recursively as follows: for each i, let aj G 9{Vi) such that < • • • < 03 < 02 < oi
and lim„_+oo On = 0 Let {x„} C X be a sequence of distinct preimages under g such
that Xj € Vj. Then the sequence {x„} converges to p and may be considered as a
discrete set in X.
Let m > 1 be given and let Wi, W2, . . ,Km be distinct free ultrafilters on the
sequence {x„}. Define Pi = {f e C{X) .3AGUi,AC Z{f)}, for each i. DeMarco
shows that each of these sets is a prime sideal of C(A") and that Mp is the sum
of any two. Thus these prime ideals are noncomparable. We will demonstrate, as
DeMarco has done for m = 2, that Mp is the sum of these m noncomparable prime
ideals.
Let {Aj}J^i be a collection of m pairwise disjoint subsets of {x„} such that
Ai eUi, for each 1 < i < m. If Bj = g{Ai), then Bi U {0} is a closed subset of R
Thus Bi U {0} is a zeroset of R Choose cpi e C(R) for each 1 < i < m such that
Z(^i) = 5i U {0} and Ejli V'i = 1r
Let «i = (fig. Then A^ C Z{g) = Z{ui), hence Uj G Pj. Finally, we have
g = Ui+U2 + f «„. If /i G Mp, then Z{h + g) = {p} and Z{h+ + g) = {p} By
the above, h+ + g,h+ge YT=i Pi and hence /i+, h,h € YlT=i Pi
The final statement of the proposition follow from the previous proposition,
since the cardinality of y^N is 2*, by [GJ, 9.2]. ■
Now that we have illustrated the extreme cases of 0,1 and infinite rank, we
present a result of [HLMW, 3.1], which gives a test for finite rank of a point of a
compact space.
31
Proposition 2.3.5. Let X be a compact space. Then p e X has rank n < oo if
and only if there exist n pairwise disjoint cozerosets {Uj}^^^ with p 6 0^=1 ^(^i)'
and no larger family of pairmse disjoint cozerosets has this property.
An /ring A is called an SVring if A/P is a valuation domain (i.e. principal
ideals are totallyordered) for every prime ideal P. A space X is an SVspace if
C{X) is an SVring. We will discuss this class of rings in more detail in Chapter 3.
However, using the above, it is shown in [HLMW, 4.1] that any compact SVspace
has finite rank. The validity of the converse of this result is unknown. Presently,
our objective is to show that the result in Proposition 2.3.5 does not hold for infinite
rank. To demonstrate this, we define a cardinal function on compact spaces and
compare its value with a known cardinal invariant.
Definition 2.3.6. Let k be a cardinal and let X be a compact space withp € X. Let
{Ua)a<K be a family of pairwise disjoint cozerosets of X and call the set na</c <^(^a)
a Kboundary. Define p{p, X) to be the infimum over all (infinite) cardinals k such
that p is not contained in a Kboundary and let p{X) be the supremum over all the
points p €: X of the cardinals p{p, X).
Recall that the cellularity of a space X, denoted c{X) is the infimum over
all (infinite) cardinals k such that every family of pairwise disjoint open sets of X
contains at most k many sets.
Proposition 2.3.7. Let X be compact. Then p{X) < c(A')+.
Proof: If p(X) > c(X)'*", then there exists a /cboundary of cardinality greater
than the cellularity of the space, which is nonsense. ■
Example 2.3.8. Let r > \!<q. The product space, 2'', where 2 is the twopoint
discrete space is called the Cantor space of weight r. We show in Example 4.3.4
32
that every point of 2"^ has infinite rank, thus rk(C(2'')) > 2* by Proposition 2.3.3.
By [E, 3.12.12(a)], we have that c+(2^) = i^j. Thus, Proposition 2.3.7 gives us that
Ko < p(2^) < i^i. Therefore, since Ki ^ 2*, we see that /9(2^) / rk(C(2^)). D
2.4 Rank via Z^Irreducible Surjections
We must first recall a few definitions from [Ha] and [HVW2]. Let X,Y be
Tychonoff Hausdorff spaces. Let / : F — > X be a surjective continuous map. Then
/ is perfect if it is a closed map such that the inverse image of any point is compact.
A perfect map is irreducible if proper closed sets of Y map to proper closed sets
of X. The pair (F, /) is a cover of X if / is a perfect irreducible surjection from
F to X. Let (Fi,/i) and (F2,/2) be covers of X. We define (Fi,/i) ~ (Fa, /a) if
there exists a homeomorphism g : Yi ^ Y2 such that /ay = /i. Order the set of
^equivalence classes of covers via: (Fi,/i) < (Fa, /a) if and only if there exists a
continuous map </ : Fi > Fa such that f^g = /i. A class of spaces C is a covering
class if for any space X there exists a least cover (F, /) of X such that Y ^ C.
The minimal extremally disconnected, basically disconnected and quasiF covers of
compact spaces are described in [PW], [V], and, respectively, in [DHH], [HudP], and
[HVWlj.
Certain covering maps allow us to compute the rank of a space externally.
A perfect irreducible surjection ^ : F ^ X is Z* irreducible if for each cozeroset
C/ C F, there is a cozeroset V CX such that dviU) = c/y(0~^(F)). This condition
on maps is also known as sequential irreducibility and ojiirreducibility. It turns out
that a map (f> is Z*irreducible if and only if C(^) is a rigid embedding of C(X) inside
C{Y), by [HaM, 2.2]. Hence we have a homeomorphism Min(C(F)) ^ Min(C(F))
via contraction, by [CM, 2.3]. It is therefore not surprising that these maps are
useful for calculating rank.
33
Example 2.4.1. Let X be a compact space. The quasiF cover of X, {QFX,<l)x),
constructed in [HVWl] has the property that (f>x is Z*irreducible. We summarize
this construction.
Let Z*{X) = {clxintxiZ) : Z G Z{X)}. For A 6 Z*{X) denote the set
of ultrafilters on Z*{X) containing A by A. The authors of [HVWl] show that
T{Z*{X)), the compact space of ultrafilters on Z*{X) whose topology has a closed
base given hy {A : A e Z*{X)},is quasiF and define a perfect irreducible surjection
(f>x : T{Z*{X)) ^Xhyae T{Z*(X)) maps to the unique point in n{A :Aea}.
Then QFX = T{Z*{X)) with the map (^x is the quasiF cover of X.
The map 0x is Z*irreducible: It is shown in [HVWl, 2.9] that if we have
A e Z*{QFX), then <l)x{A) € Z*{X), which is equivalent to the property of Z*
irreducibility. In fact, the quasiF cover of X is characterized up to equivalence
in [HVWl, 2.13] as the only cover {Y,f) of X for which Y is quasiF and / is
Z*irreducible. D
Before we continue, we discuss the question (now answered) which was our
motivation for considering this line of investigation. Let A^ be a compact space of
finite rank and W = {x e X : Tkx{x) > 1}. In [La2], Suzanne Larson asks if W is
always closed in X. The answer is no. Her counterexample, presented at ORD98 (a
conference on ^groups held in Gainesville, FL in 1998) follows:
Example 2.4.2. Let W be a free ultrafilter on N. Let E = N U {a} where points
of N are isolated and neighborhoods of a are of the form U U {a}, where U eU.
Let Ej = E for j = 1,2 and define F = (Ei U E2)/(<7i ~ ^2). Let Yr = Y for each
r € R, and let X = (JJ^gR Yr) U {00} where neighborhoods of 00 contain all but
countably many copies of the Yr. Then 00 is a Ppoint which is in the closure of the
setW = {xeX : ikxix) > 1}. D
34
We provide a characterization of points of finite rank of a compact space X
via Z*irreducible maps onto X. Let B be an /subring of A and let 9 denote the
natural surjection Max(A) > Max(J3). The following is proved in [HaM, 2.5].
Lemma 2.4.3. Let A and B be commutative f rings with identity and hounded
inversion. Let B he an fsuhring of A. Then if B is rigid in A, we have that
0{eM) = n{0{N) nB .ON = OM}, and if 9Ni = OM (for j= 1,2) with Ny \\ N2,
then {0{Ni) n B) \\ (©(iVa) n B).
Let X and Y be compact spaces and (f) .Y ^ X a. Z*iiTeducible map. Then
e : Max(C(y)) ^ Max{C{X)) is given by M »> {/ € CiX) : f(l> e M}.
Lemma 2.4.4. Let X and Y be compact and (f) : Y ^ X a Z* irreducible map.
Let p G X then 0{Mg) = Mp if and only if <j>{q) = p. Therefore, we have that
Op = n{0, n (7(X) : g e r Hp}}
Proof: Let q e 4>~^{p}. If / € ^{^q) then /(^ 6 M, and therefore we have that
= /^(9) = /(p) and / e Mp. Let g G Mp, then {g(f>)iq) = gip) = 0, and hence
g<j> e 0{Mg). Thus 6{Mg) = Mp. Conversely, assume ^(g) = r ^ p. By complete
regularity, there exists / € C{X) such that f{p) = and /(r) = 1. We have
/ 6 Mp, but /0(g) = /(r) = 1^0 and hence / ^ 6{Mg). The final statement then
follows firom the first and Lemma 2.4.3. ■
Proposition 2.4.5. LetX andY he compact spaces and<f> .Y ^ X Z* irreducible.
1. Ifp e X such that rkxip) = n, then \(f>~^{p}\ = n.
2. IfYis an Fspace and \(j>~^{p)\ = n then rkx(p) = n.
S. IfY has finite rank and \(f>~^{p}\ = n, then rVxip) < 00. Explicitly, we have
that rkx(p) = EILi rky(ft) where (j>^{p} = {ft}?^i.
35
4 IfpGX is an Fpoint of X, then q e (f>~^{p} is an Fpoint ofY.
Proof: (1) Let <f>~^{p} = {qi}iei for some index set /. For each i e /, choose
Qi e Mm(C(F)) such that O,, C Qi. Then by Lemma 2.4.4, it follows that we
have Op = HieiiOg, D C{X)) C Qj n C{X) C Mp, for every i e /. By the bijection
described in Proposition 2.2.7, the set of minimal prime ideals contained in Mp is
given by {Qi n C{X)}i^i and / = n.
(2) Let <f>~^{p} = {qi}u=i If 5^ is an Fspace, then Og^ is prime for each i and we have
that Op = n;Li(0,. n C{X)) C O,. n C{X) C Mp for each i. Thus, by the bijection
described in Proposition 2.2.7, the subspace of minimal prime ideals contained in
Mp is {Og, n C(X)};Li and xkxip) = n.
(3) Let ^~^{p} = {9j}iLi and let the subspace of minimal prime ideals in M, be
given by {Qi^ '■ l < j < rkY{qi)}^=,i Then as above, the subspace of minimal prime
ideals contained in Mp is given by {Qi. D C{X) :l <j < rky(9i)}"=i and hence we
see that rkx(p) = Yl^=i ^^viQi) < oo, as desired.
(4) Let p G X be an Fpoint and <^~Hp} = {q} Then Op = O, n C{X) C Mp is a
minimal prime. There exists a unique Q e Min(C(F)) such that Op = Qr\ C{X).
Since O, n C{X) = Qn C{X) and Q is unique, Og = Qe Min(C(y)). ■
If K = QFX in Proposition 2.4.5, then the third statement is a partial
converse of [HLMW, 5.1] which states that if X is compact and has finite rank then
QFX has finite rank. The final statement is an extension of [HVWl, 3.12] in which
it is shown that the preimage of a Ppoint of X is a Ppoint of QFX. The second
statement says that if QFX is an Fspace, then the points of X of rank one are
precisely the points with unique preimage under the covering map ^x In this light,
one should ask when a quasiF cover of a space is an Fspace.
Recall that a space X is fraction dense if the classical ring of quotients of
C{X) is rigid in the maximal (Utumi) ring of quotients of C(A'). In [HVWl, 2.16],
36
it is demonstrated that QFX is basically disconnected if and only if X is cozero
complemented. In fact, the basically disconnected cover is the quasiF cover in this
case, see [HaM, 2.6.2]. The fact that QFX is realized by the extremally disconnected
cover if and only if X is fraction dense is proved in [HaM, 2.4]. By [HudP, 6.2], QFX
is an Fspace if and only if for any two disjoint cozerosets Ci, Ca C X, there exist
Zi, Z2 e Z{X) such that d C Zi for i=l,2 and int{Zi n Z2) = 0.
We now provide an example of a space X such that QFX is an Fspace which
is not basically disconnected. Recall that a space is acompact if it is a countable
union of compact spaces.
Lemma 2.4.6. Let X be a noncompact acompact locally compact Fspace which
is not basically disconnected. Let Xi = X2 = PX and define Y be the quotient
space of the topological sum of Xi and X2 where pairs of corresponding points of
Xj \ X (j=l,2) are collapsed to a single point. Then Y is not quasiF and QFY is
an Fspace which is not basically disconnected.
Proof: The disjoint union U = X II X is a. dense cozero set of Y which is not
C*embedded in Y. Thus Y is not quasiF. The quasiF cover of Y is Xill X2,
which is an Fspace but not basically disconnected. ■
Example 2.4.7. Let X be the disjoint union of a countable number of copies of the
corona jSN \ N. Then X is a acompact noncompact Fspace which is not basically
disconnected. Construct Y as defined in Lemma 2.4.6. Then QFY is an Fspace
which is not basically disconnected. D
2.5 Prime Character
The second character we consider counts the minimum number of minimal
prime subgroups that we must sum in order to obtain the lex kernel of an ^group.
37
Definition 2.5.1. Let S be a family of minimal prime subgroups of G. We call S
ample ifVS = lex(G). The prime character ofG, denoted '^{G), is the least cardinal
so that any family of distinct minimal prime subgroups of that cardinality is ample.
Note that i/lex(G) = 0, i.e., if G is a totallyordered group, then we say n{G) = 1.
A prime subgroup properly contained in lex(G) is called embedded.
Proposition 2.5.2. Let G be an igroup and m a positive integer. The following
are equivalent:
1. 7r(G) = m < oo.
g. m = 1 + sup{rk(P) : P € Spec+(G) is embedded}.
3. m is minimal with respect to the property that lex(G) = G(U]^^aj') for any
m pairwise disjoint positive elements, {aj}^i C lex(G)"'".
4 m is minimal such that for any embedded prime P, the chains of proper polars
in P have length at most m—1.
Proof: (1) =^ (2) : Let P be embedded. If P contains the m minimal prime
subgroups {Qj}]Li C Min+(G) then V^^Qj C P C lex(G). Hence 7r(G) > m. Thus
we have shown that 7r(G) < m implies rk(F) < m. Thus by (1), rk(P) < m  1 and
sup{rk(P) : P embedded} < m  1. If sup{rk(P) : P embedded} < m  1, then for
any family 5 of m  1 minimal prime subgroups of G, yS is not embedded since
rk(v5) > m 1. Thus V^ = lex(G) and 7r(G) < m 1. Thus ir{G) = m implies that
sup{rk(P) : P embedded} > m  1. Therefore sup{rk(P) : P embedded} = m + 1.
(2) =j> (3) : Let {oj}^! C lex(G)+ be pairwise disjoint. Let g G lex(G) \ GdJjt^ af )
and let V be a value of g such that G(U!^i af) CV C lex(G). Then by the polar
characterization of 0(y), [BKW, 3.4.12], we have that Oj ^ 0{V) for each j such
that 1 < i < m. By (2), V contains at most m  1 minimal prime subgroups,
{Qi}7^i^ Since 0{V) = fll^"^ Qi, we have by the pigeonhole principle that there
38
must be a minimal prime subgroup, Q, contained in V which fails to contain two of
the elements of {aj}f^i. But since these elements are pairwise disjoint, this contrar
diets the fact that Q is prime. Therefore, lex(G) = G(Uj° ^ a^).
For the minimality of m, let P be an embedded prime of rank n < m.
Then by the Finite Basis Theorem, P/0{P) contains n pairwise disjoint elements
of corresponding to elements of P which are not in 0{P), say {&*})t=i C P. Then
6j^ C F for each A; such that 1 < A; < n and G(ULi ^it") ^ ^ S lex(G).
(3) =^ (1) : Assume that 7r(G) > m. Then there exists S = {Qj}f=i C Min+(G)
which contains m elements and is not ample. Let Q = VS ^ lex(G). For each
j, let Qij € Qt \ Qj. Then ft = Vf^^Qij e Qt \ (Uk^iQk) Disjointify by defining
ql = Ai^jQj — AjJLigfc ^ Qi. Then q] €^ Qt for every j ^ i and then we obtain that
G'dJ^i Qj^) C V5 g lex(G). Hence, 7r(G) < m. By the minimality of m in (3) and
by (1) =>■ (3), we must have 7r(G) =m.
(3) «> (4) : Follows directly from the Finite Basis Theorem [D, 46.12] applied to
P/0{P) for any embedded prime P. ■
The following is immediate:
Corollary 2.5.3. Let G he an igroup. The follovnng are equivalent:
1. 7r(G) < 00.
2. sup{rk(P) : P embedded} < oo.
3. There exists m G N such that lex(G) = Gi\J^^^ af) for all families of m
pairwise disjoint elements {aj}^i.
We now consider some ^grouptheoretic properties of the prime character.
Note that for ^groups A and B, A^B denotes the ^group AxB with componentwise
operations and is called the cardinal sum.
39
Proposition 2.5.4. Let G he an igroup.
1. For any £homomorphic image, H, we have i^{H) < 7r(G).
2. For any C E C{G), 7r(C) < 7r(G). //7r(G) < oo, then 7r(C) = 7r(G) if and only
if C contains every minimal prime subgroup of G.
3. If'K(A),7r{B) <oo and G = ABB, then 7r(G) < n{A) + n{B)  1.
Proof: (1) Let (p : G ^ H he aji ^surjection with kernel K. Then by [D, 9.11],
the prime subgroups of H correspond to prime subgroups of G containing K. Thus,
by the characterization of prime character in terms of ranks of prime subgroups, we
have that 7r{H) < 7r(G).
(2) This result follows from [BKW, 2.4.7] and [D, 27.8].
(3) Let G = AmB and let P be a prime subgroup of G. Then P = (PnA)ffl(PnB),
by [D, 27.8]. Hence P contains at most (m — 1) + (n — 1) = m + n — 2 minimal
prime subgroups and therefore 7r(G) < m + n — 1, as desired. ■
Recall from [D, 36.1] that a class C of ^groups is a radical class if G e C
implies the following:
1. ^{G)CC,
2. every ^isomorphic image of G is in C, and
3. if {Ax}xeA C (tiG) n C, then VacaAa e C.
■1I;:; ;^' ■ ■■. i^\.. '■■■
In view of Proposition 2.5.4, it is natural to ask if the class of all ^groups of finite
prime charax;ter is a radical class. The answer is no; the following is a counterexam
pie.
Example 2.5.5. We construct an ^group with two convex ^subgroups, A and B,
each of finite prime character such that the supremum Av B has infinite prime
■^•i. j ■
40
J i
? • • i >■
chaxacter.
Lb
(2.1)
Let r be the root system above, where the subgraphs Ta and Tb each have infinitely
many identical branches descending from its maximal vertex. Define G = E(r, R),
A= {v eG : supp(u) C Ta} and B == {v e G : supp(v) C Tb}. Then we have that
A ^ S(r^,R), B ^ E(rfl,R) and ^ V B ^ ^{Fa U Tb,^) Now, it is evident that
7r(G) = 00, it{A) = 3 = 7r(B), and Tr{A V B) = oo. D
Recall that an ^group G has a finite basis if it contains a finite maximal set
of elements {bj}^^i such that the set {g € G'^ : g < bj} is totally ordered for each
j. The foUoAving indicates when we can expect 7r{Ay B) < oo:
Proposition 2.5.6. Let G be an igroup and let A, B be convex isubgroups such
that iriA) = m < oo and ■k{B) = n < oo. //lex(A V B) = lex(A) V lex(B), then
'k{A\/ B) < 00. Otherwise, it{Av B) < oo if and only if each of A and B has a
finite ba^is.
Proof: Assume that lex(AvB) = lex(A) Vlex(B) and let P g lex(AVB) be a prime
subgroup of G. Then either lex(A) ^ P or lex(P) ^ P, or both. Say \ex{A) ^ P.
Then P n lex(>l) is an embedded prime subgroup of A, and since ^{A) = m, we
have that rk^(P n lex(A)) < m  1. Then by [BKW, 2.4.7], rk^vB(P) < m  1.
Likewise, if lex(B) ^ P then rkyivB(P) < n  1. Thus, for every prime subgroup
P g \ex{A V B), we have that rk,i vb(P) < max{m l,nl}<oo. Therefore, by
Proposition 2.5.2, tt{A V P) < oo.
41
Note that we always have that lex(A) V lex(B) C lex(^ V 5). We assume
now that lex(A V B) ^ lex(A) V lex(B) and let P e Spec+(A V B) have the property
that lex(A) V lex(B) C P g iex(A V B). If each of >l and B has a finite basis, then
rk>ivB(^) = Min+(^) + Min+(J5) < oo and for any embedded prime subgroup Q
ofAVB, we have that iIcavbCQ) < Min+(A) + Min+(B). Thus, by Corollary 2.5.3,
we have that iriAs/B) < oo. Conversely, 7r(AvB) < oo implies that rkAvB(^) < oo.
Hence, since Min+(A), Min+(B) < rk^vB(P)) we have that each of A and B has
a finite basis by the Finite Basis Theorem. ■
The proof of the following is evident:
Proposition 2.5.7. Let G be an igroup and let A,B E €(G). If A C B = lex(B)
or iflex{A) = A and lex(B) = B, then \ex{A V B) = \ex{A) V lex(5).
We now compare the property of finite prime character to Conrad's Property
F: every element g 6 G"*" exceeds at most a finite number of disjoint elements. The
following is compiled in Conrad's Tulane Notes, [C]:
Proposition 2.5.8. Let G be an (.group. The following are equivalent:
1. G has Property F.
2. Every bounded disjoint set in G is finite.
3. For every element g E G, the convex (subgroup G{g) has a finite basis.
4 Every element of G is contained in all but a finite number of minimal prime
subgroups.
Corollary 2.5.9. Let T be any root system in which each maximal element lies
above a finite number of minimal elements. Then E(r,R) has Property F.
Proof: Let v 6 E(r, R). Let C be a maximal chain in F and let the associated
minimal prime subgroup be He = {v e E(F,R) : 7 e C =^ z;(7) = 0}. U v ^ He,
42
then there exists 7 e C such that ^(7) 4" 0 Since supp(t;) is finite, v is in all
but finitely many minimal prime subgroups of S(r, M). Thus by Proposition 2.5.8,
E(r, R) has Property F. ■
Proposition 2.5.10. //G is a finitevalued £ group of finite prime character, then
lex(G) has Property F.
Proof: Let g G lex(G). Then any minimal prime subgroup of lex(G) not containing
g is contained in a value of g. Since each value of g contained in lex(G) contains a
finite number of minimal prime subgroups and there are only finitely many values
of g, there are only finitely many minimal prime subgroups of lex(G) not containing
g. Thus lex(G) has Property F. u ,
Proposition 2.5.11. Let G be an igroup and let m be a positive integer. If m
is minimal such that every pairvnse disjoint subset of G contains at most m — 1
elements, then 7r(G) = m.
Proof: Any proper prime subgroup of G is contained in a value of G, hence proper
prime subgroups of G contain at most m — 1 minimal prime subgroups. Thus
7r(G) < m. Let n <m and assume that every family of n minimal prime subgroups
of G is ample. Then every proper prime subgroup (in particular, every value) of G
contains at most n  1 minimal prime subgroups. This contradicts the minimality
of m. Thus 7r(G) = m. ■
Example 2.5.12. The following is an example of an ^group of finite prime char
acter such that lex(G) has Property F but G has pairwise disjoint sets of any size
m. Thus the converse of Proposition 2.5.11 does not hold.
Let r be the root system:
(2.2)
43
and let G = E(r,R). Then n{G) = 3 and G is finite valued. Thus, by Proposi
tion 2.5.10, lex(G) has Property F. We demonstrate that there are bounded disjoint
families of any given size.
Index the maximal elements of T by fj.j, where j is a positive integer, and let
Vj e G be such that Vj{fij) — 1 and supp(uj) C {7 6 P : 7 < /ij} where Vj{6) = for
all (J < Hj. Then {vj}j>i is an infinite pairwise disjoint family in G. Let a positive
integer m be given. Choose any m elements from this set, {vj,, Vj,, . • • , Uj„} and let
S = UfcLiSupp(%). Let V be the characteristic function on the finite set S, then
« € E(r,R) and u > Ujj for A: = 1,2, . . .m. D
Example 2.5.13. The converse of Proposition 2.5.10 does not hold. That is, we
present an example of a finite valued ^group with Property F and infinite prime
character. Consider the following root system F which is indexed by the positive
integers:
A A
(2.3)
Then each prime subgroup of the ^group G = E(r, R) contains a finite number of
minimal prime subgroups, yet there is no bound on the number of minimal prime
subgroups in each prime subgroup. Thus 7r(G) = 00. G has Property F by Corollary
2.5.9. D
2.6 Filet Character
The third character that we define measures the length of a chain of incidents
of branching.
Definition 2.6.1. Let G be an tgroxip. C = {Pi,Qj G Spec+(G) : i > 0, j > 1} w
called a filet chain of prime subgroups i/ Po 2 A 2 ^2 • • • , for all i, Pi  Qi, and
44
Pi+i V Qi+1 C Pi for alli>Q (see below).
(2.4)
The length 0/ a filet chain is given by 1{C) = max{j G N : 3Qj G C}. // the
maximum does not exist, we write 1{C) = 00. The filet character ^{G) is given by
<I){G) = sup{/(C) : C is a filet chain}. //Spec+(G) has no filet chains, i.e., if G is
semiprojectable, then we say that <I>{G) = 0.
Proposition 2.6.2. Let G be an igroup. Then <i>{G) <l if and only if7t{G) < 2.
Proof: Suppose that 7r(G) < 2. If (f>{G) > 1 then there exists a filet chain C of
length 2 in which we may assume that Pq = lex(G). Since rk(Pi) = 2, we have
that 7r(G) > 2, by Proposition 2.5.2. Conversely, assume that 7r(G) > 2. Then
there exist minimal prime subgroups P2, Q2 such that Pi = P2 V Q2 / lex(G). Thus
for any minimal prime subgroup Qi ^ Pi, the set C = {Pq,Pi,P2,QuQ2}, where
Pq = lex(G), is a filet chain of prime subgroups in G of length 2. Therefore <I>(G) > 1.
m
For larger filet character, the relationship between it and the prime charac
teris more complicated, as the following example illustrates.
Example 2.6.3. Let F be the following root system:
(2.5)
45
Then G = E(r,R) has <^(G) = 2 while 7r(G) = 4. D
The following relationships between the characters hold:
Proposition 2.6.4. Let G be an igroup. Then
1. <^(G) < rk(G)  1.
2. (I>{G) < n{G)  1 < Tk{G).
Proof: Let rk(G) = m. If G has a filet chain C = {Pj, Qj e Spec+(G) : i > 0, j >
1}, then rk{Po) < m and hence 1{C) < m 1. Therefore, <f>{G) <ml= rk(G)  1.
If 7r(G) = n and C = {PuQj 6 Spec^(G) : i > 0,j > 1} is a filet chain
in G then 1{C) < oo since rk(Pi) < n — 1 by Proposition 2.5.2. Hence, in fact,
1{C) < n  1 and therefore, 0(G) < n  1 = n{G)  1. Now, 7r(G) < rk(G) + 1 by
Proposition 2.5.2. Thus, finally, 0(G) < 7r(G)  1 < rk(G). ■
At this time, any statement that we make about the filet character of an
^group requires a restriction on the rank and prime character. Rather, we can
not say much more than what we establish earlier in this chapter. We leave this
investigation for a later date.
'■ ^ ■»;
CHAPTER 3
GENERALIZED SEMIGROUP RINGS
Let A be a root system. Starting from the standard semigroup ring and Hahn group
constructions and the investigation of the following section, we build an /ring H
having A as the skeleton of the graph of Spec^(7i). We follow up on ideas presented
in two papers of Conrad [Cl],[C2], in the paper of Conrad and Dauns [CD], and
in the paper of Conrad and McCarthy [CMc]. The first two papers look at the
conditions on A which will yield a ring structure on V{A, R) and on the subgroup
F{A, R) of elements v for which supp(u) is the join of finitely many inversely well
ordered sets in A. The paper [CD] focuses on the case when V(A, R) is a division
ring, while in [CMc] the conditions are established for the ring to be an ^ring its
properties are studied when A is finite. Note that F(A,R) is denoted by W{A,R)
in [C1],[C2], and [CD].
3.1 Specially Multiplicative /Rings
Let r be a partially ordered group which is a root system. Suppose also that
r is torsion free, that the subgroup H ofT generated by the positive cone is totally
ordered and that T/H is finite. In the paper of Conrad and Dauns [CD, 2.2], it is
shown that V{T, R) = F(r, R) and that V{r, R) is a latticeordered division ring
under the usual groupring multiplication: for u,v E V(r, R), and 7 € F
u * v{'y) = yj u{a)v{l3).
It is easy to see that if an element of (V"(r,R),,*) is special (i.e., has only one
maximal component), then its multiplicative inverse is also special. Hence, the
46
47
special elements of the ring y(r,R) form a multiplicative group. Moreover, the
following holds in general, [CD, Theorem I]:
Theorem 3.1.1. Let R be a latticeordered division ring with identity. The follow
ing are equivaient:
1. The special elements of R form a multiplicative group or the empty set.
2. Ifa^R^is special, then a~^ > 0.
3. Ifa^R is special, then a~^ is special.
4 For all a e R^ special and x,y e R, a{x V y) = a:cV ay.
Since the authors of the paper [CD] seek an embedding theorem for ^fields,
their investigation in this realm is restricted to division rings. In this section, we
consider a class of /rings (which are not division rings) in which the special values
form a partially ordered semigroup. The pursuit of a characterization similar to
Theorem 3.1.1 of ^rings satisfying this condition is left for another time. For the /
rings that concern us here, some particular assumptions are needed on the associated
semigroups. As a formality, we define:
Definition 3.1.2. Call an fring A specially multiplicative if the special values of
A, with an appropriately adjoined 0, form a partially ordered semigroup.
Recall that an /ring A is called an SVringifA/P is a valuation domain (i.e.,
the set of principal ideals is totally ordered) for every prime ideal P. Let G be the
class of commutative /rings which are local, bounded (that is, A* = A), semiprime,
finite valued, finite rank and square root closed SVrings with identity and bounded
inversion. We demonstrate that the elements of Q are specially multiplicative and
then investigate the properties of the associated semigroups. We must first remind
the reader of a couple of facts about special values and of the relatively deep theorem,
recorded as [HLMW, 2.14]:
48
Theorem 3.1.3. Let A be an fring of finite rank with identity and bounded inver
sion which is local, semiprime and square root closed. Then A is an SVring if and
only if whenever < a <b and b is special, there exists x ^ A such that a = xb.
The proof of the following lemma is wellknown and routine, however, it is
instructive, so we include it here. We remind the reader that we denote the root
system of values of an ^group G by r(G) = {Vj : 7 6 F}, where T is a partially
ordered index set that is orderisomorphic to r(G).
Lemma 3.1.4. Let G be an (.group and let a,b ^ G^ be distinct special elements
with values atVa,Vfi, respectively.
i Va II Vff if and only ifaAb = 0.
2. Va < V^ if and only ifa<^b.
Proof: First, assume that Va \\ Vp. We show that a A 6 is contained in each value
of G and conclude that a and b are disjoint. Let V be a value in G and assume
that b ^ V. Then V C V^ and a e V since, else, V C Va, which contradicts the
assumption of incomparability. Thus, by convexity we obtain that aAbGV. Again
by convexity, if b e V then a A b € V. Thus a Ab = 0, as desired. Conversely,
assume that a and b are disjoint and, by way of contradiction, assume (without loss
of generality) that V^ < V^. Since a ^Va and K» is a prime subgroup, b ^ Va < V^,
a contradiction.
Second, assume that Va < V^. Then a e V^. If there exists an integer n such
that na > b, then since 6 ^ V/j, we must have that no ^ V^, by convexity. But this
contradicts that a e F^, and so we conclude that a <C 6. Conversely, assume that
a < 6. We know that b^Va and hence Va < V^. If Va = V^, then by [D, 12.6], there
exists an integer n such that na > b, which is nonsense. Thus K* < V^. ■
49
Combining Theorem 3.1.3 and Lemma 3.1.4 leads to the main result of this
section:
Theorem 3.1.5. Let A^Q.
1. // a, 6 6 A^ are special and not disjoint, then ah is special.
2. If a, a' are special with value Va, b, b' are special mth value V^ and Va,Vff are
comparable, then the special value of ab is the same as that of a'V.
Proof: Let a,6 € .A+ be special with values at Va^V^, respectively, and let M
be the maximal ideal in A. Since A is bounded, we may assume without loss of
generality that Va, V^ < M. This gives rise to the relations a, 6 < 1 and ab <C a, by
Lemma 3.1.4(2).
Theorem 10.15 of [AF] states that an abelian ^group is finite valued if and
only if each positive element is a finite disjoint sum of positive special elements.
Thus we assume, by way of contradiction, that ab = ci + C2 + he*, where each
Cj e A"*" is special and Cj A Cj = for alH / j. Then < Cj < a6 < a for each i
and therefore Theorem 3.1.3 gives the existence of Xj e A'^ such that Cj = axi for
each i = l,...,k. Without loss of generality, we may assume that < Xj < 6, by
replacement with Xj A b. Assume that Xj = YllLi ^ij ^ ^ decomposition of Xj into
a sum of special elements. Then Cj = Y^Li o^i, implies that axj^ = for all but
one j, by [AF, 10.15]. Thus we may choose each Xj to be special, without loss of
generality.
For each i, let V^^ be the value of Xj. Then 6 ^ Vs< and therefore for all i, we
have that F^. < V^. If 1^. < V^, then Xj <^ b. This gives a contradiction, since it
implies that oxj < ab. On the other hand, if V^. = V^, then there exist n, m G N
such that Xj < nh and 6 < mxj. Therefore, axj < nab and ab < maxi. Thus, by [D,
12.6] we have that the value of Cj is the unique value of ab and ab = axj. Hence, ab
is special.
50
For the second statement, let Vs be the value of the special element ab. We
show that Vs is the value of abf and then a similar argument and transitivity gives
that Vs is also the value of a'b', as desired. Towards this end, let V^ be the value of
ab'.
If V^ II Vs, then = ab Aal/ = a{b A b') and hence = a A (6 A ?/). On the
other hand, if Va < V^, then since bAbf ^Va,'we have that a e Va If V^ < Vq,
then since a ^ V^, we have bAb' and hence 6 or ?/ is in VJj. These contradictions
lead us to conclude that V* < Vy or V^ < Vs. If Vs < Vy then ab < ab'. However,
by [D, 12.6], there exists an integer n such that nb > H and therefore, nab > naii .
Likewise, V^ ^ Vs. Thus V^ = V^, as desired. ■
Let i4 6 ^. Then by [AF, 10.10], all the elements of Y{A) are special since
A is finitevalued. Abusing notation, we now identify values with their indices and
define an operation on P. Let a, 6 e A be special elements with corresponding values
at a,/S, respectively. Append <^, a generic symbol, such that (^  a for all a G F and
define a multiplication • on F U {^} such that <f)a = <!) = a <f> and
/<?i if a
I the value of ab oth(
\\P,
otherwise.
By Theorem 3.1.5, this operation is welldefined. Some properties of this operation
are recorded in the following proposition. Define a ~ /? if and only if a and /? are
contained in the same maximal chains of F. Then ~ is an equivalence relation on F.
Let Fq denote the ^equivalence class of a.
Proposition 3.1.6. Let AeQ and let a,j3,'y,5 gT correspond to the values of the
special elements a,b,c,d € A"*", respectively. Then
1. The operation is associative, for every a, /?, 7 : (a ■ /?) • 7 = a • (/? • 7)
2. If M is the maximal ideal of A, then M is a value of the identity. Let fi = M.
Then fi a = a fj. = a for every a < fi.
51
3. Ifap^(l)thena'l3<a,l3.
4. Ifa<(3 and 7 is comparable with a, then a • 7 < /? • 7 and 7 • a < 7 • /?.
5. // a / /i, then a • a < a.
6. Maximal chains in F are closed under the operation.
7. For each a € F, the equivalence class Fa is closed under the operation.
8. If'y< a, then there exists /3 G F such that 7 = a • /?.
Proof: (1) Note that the products under consideration are all (f) if any of the factors
multiplied are actually (^. If a  )9 then aAb = 0. Thus aA&c = and hence a  /?7
which implies that (a • /?) • 7 = ^ • 7 = ^ = a • (/9 • 7). Likewise, if ;5 • 7 = then
both products are equal to <j).
Assume that a • /? ^ (^ and /? • 7 / <^. Then either (a • /5) • 7 = or it is the
value of the special element {ab)c = a{bc). Thus, it suffices to show that a • /?  7 if
and only if a  ;9 • 7. If a  /5 • 7 then a A 6c = and hence = acAbc= {aAb)c and
so = (a A 6) A c. Therefore, for any value 7/ of a A 6, we have tj \\ 7. But a 13 <r},
so a • /? II 7. The converse follows similarly.
(2) Since A is bounded and has bounded inversion, the maximal ideal is a value
of the identity, by [Wo, 2.3.4]. Hence, it follows that // • a = a • // is the value of
la = al = a, namely a.
(3) Assume, without loss of generality, that /3 < a < n. Since ab •< b, we obtain
that a •/?</?< a. If /?< a = /i, then a /3 = fjt /3 = p <a,hy (2).
(4) If a < /9 then a ■< 6 and thus ac '^ be and ca C cb. By Lemma 3.1.4(2), this
just says that a • 7 < /? • 7 and 7 • a < 7 • /3, as desired.
(5) Since a^ C a, we know that a • a < a.
(6) Assume, without loss of generality, that /? < a. Let 6 < f3he such that 6 \\ a p.
Then abAd = and < d <C 6. Thus by Theorem 3.1.3, there exists y € A such that
52
d = yb. Then = abAyb= {aA y)b gives that = a A y A 6 and hence = ayb = ad
and therefore aArf = 0. This says that a  (J, a contradiction. We now have that a/?
is comparable with any 5 < ^.It follows from this fact and from (3) that maximal
chains are closed under the operation.
(7) Let 7, (J G Fq. By (6), we have that 7 • (5 is in the same maximal chains as 7 and
6. Hence 7 • 5 e Fq.
(8) We know that 7 < a if and only if Vy < Va if and only if < c < a. This implies
that there exists a positive element b such that c = ab, by Theorem 3.1.3. We have,
by [AF, 10.15], that b is special. Thus, if V^ is the value of b, then 7 = a • ^5, as
desired. ■
Corollary 3.1.7. Let A € Q. Then A is specially multiplicative. Moreover, the
operation • is a surjection onto T{A) U {<^}.
3.2 rSvstems and ^Systems
Let A be a given root sj^tem. The discussion of the previous section and
of the papers [Cl], [C2], [CD], and [CMc] prepare us to construct an /ring having
A orderisomorphic to its root system of values. However, under the assumptions
placed on A in these papers, these rings sometimes are ^rings, and rarely are /rings;
in fact, as mentioned before, the paper [CD] focuses on the case when y(A,R) is a
division ring. Also, contrary to our intentions, [CMc] concentrates on the properties
of the ring when the root system is finite. We modify all these conditions on the
root system in order to get a ring multiplication, *, yielding an /ring structure
on 71 = (F(A,R),+,*) such that the subring 8? = (E(A,R),H,*), as defined in
Chapter 2, is an /subring. We start with [Cl, Theorem I] which establishes the ring
structure.
Proposition 3.2.1. Assume that A is endowed with a surjective partial binary op
eration I : A » A defined on A C A x A. Let u,v €. y(A,R), and define for
53
5 e A, u*v{5) = Y>a+$=s u{a)v{l3). Then both 3? and H are closed under *. In fact,
9? and H are rings if and only if the operation on A is (BaerConrad) associative:
(a,/3), (a+i3,7) e A if and only if{(3,j), (a,/?+7) ^ A, and if{a,(3), (a+^,7) ^ A
then (a + ^) + 7 = a + (/? + 7).
Certain properties of these rings, 3? and K, are completely determined by the
operation + on A; conmiutativity is one such attribute.
Definition 3.2.2. Let (A, +) be a root system with a partial binary operation, +.
We call (A, +) an rsystem if the operation is surjective and (BaerConrad) asso
ciative. An rsystem (A, +) is commutative if (a,/?) E A if and only if {f3, a) G A
and a + /? = /? + a for all such pairs.
Proposition 3.2.3. Let (A, +) be an rsystem. (A, +) is commutative if and only
iflZisa commutative ring.
Proof: If (A, +) is commutative, then for u,v EKwe have
u*v{6)= y^ u{a)v{P) = y^ v{(3)u(a) = Y^ v{(3)u{a) = v *u{S).
Thus 7i is a commutative ring. ' "' ' ^ '•
Conversely, if a + ^5 / /? + a, then Xa+^ = Xa*Xpi' Xp*Xa = X^+a ■
In order to obtain an ^ring, we must ask that the operation on A preserve
the order of the root system and restrict the domain of the operation. As we will
indicate, the various strengths of order preservation and restrictions of domain yield
varying richness of order structure on the rings.
Definition 3.2.4. Let A be a root system. An rsystem (A, +) is an ^system
if it satisfies: a < /3 and (a, 7) 6 A implies that (/?, 7) e A and, in this case,
a+7 < /?+7; and »/(7, a) e A implies that (7, f3) E A and, in this case, 7+a < 7+/?.
If every connected component of A has a maximal element, we call A bounded above.
54
Note that the ^system condition gives that no nonmaximal element of such
a bounded above root system is idempotent. The following theorem is in [CMc, §2].
Theorem 3.2.5. Let (A, +) be an rsystem. Then U is an iring if and only if A
is an isystem.
In the next section, we describe the conditions on an ^system which induce
an /ring structure on 72.. As mentioned, there are some restrictions that we need
to place on the domain and range of the operation. Before we discuss this situation,
we present an example of an ^system that gives rise to an ixmg which is not an
/ring. Probably, this is the simplest example of such an ^system.
Example 3.2.6. Let A > a > 2q; > Sa . . . and //>/?> 2/? > 3/3 ... . Then
we let Aa = {A} U {na}'^^, let A^ = {n} U {n/3}^i and totallyorder Aq x A^
lexicographically such that (^i , 71) < {62, 72) if and only if 5\ < 62 or we have 5i = S^
and 7i < 72. Identify pairs with sums and let A^^.^ = {<J+7 : {^, 7) £ A^ x A^}. We
define an associative and commutative addition on the root sj^tem given by disjoint
union A = Ac U A^ U Aa+/? as shown in the following table, where i^ = A I /x and
k, I, m, n are positive integers (note that the table is completed by reflection across
the diagonal):
+
a
A
M
V
X + ml3
ka + ii
na + l0
Q
2a
a + /9
a + /i
a + /i
a + m0
{k + l)a + ti
(n + 1)q + 10
/3
2/3
A + ^
fi
A + /9
A + (m + 1)0
ka +
na + (l + 1)0
A
A
V
V
A + m/3
ka + ^
na + l0
(t
^
V
A + m/3
ka + fi
na + l0
V
V
A + m/?
ka + n
na + l0
A + m/3
A + 2m/?
ka + m0
na + {m + l)0
ka + ii
2Jta + /i
(fc + n)a + 10
na\l0
2na + 210
Let u, u e 71'^. We need to show that u*v € 71'^ in order to conclude that Tl
is an ^ring. Since u and v are positive, it is evident that u*v{X),u*v{fj.), u*v{i') > 0.
If = u * v{X) = u(A)t;(A), then assume that u(A) = 0. For an integer n,
u * v{na) = u(X)v{na) + u{na)v(X) + \^ u{ia)v{ja).
i+j=n
55
If u(Tia) = for all n, then « * t;(na) = 0. Conversely, if n ^ is minimal such that
na € supp(w), then u * v{na) = u{na)v{X) > 0. Thus, if v{X) > then na is the
maximal element of supp(« * v) below A.
If 7;(A) = and v{Tna) — for all m, then u*v vanishes on Aq. Otherwise,
let m be minimal such that ma 6 supp(7;). Let k = n + Tn then ka is maximal in
supp(« * v) and u * v{ka) = u{na)v{ma) > 0. A similar computation demonstrates
that w * t; > at the maximal element of its support in A^ also.
Assume that = u*v{u) and, when they exist let
A„ = mox(supp(u) n Aa) A„ = max (supp(t;) n Aa)
/i„ = max(supp(u) H A^) ^ = max(supp(t;) n A^)
5rt = max(supp(u) n Aa+^) (J„ = max(supp(t;) H Aa+0) (3.1)
Then the maximal element of supp(M * v) lying below u is given by
7 = max{ Au + //u, Au + 5v, A„ + /iu, A„ + (Ju, Mw + <^t), /^o + <^tt> <5u + Sy}
and so we are left to show that u * u(7) > 0. We have three cases to check here,
namely 7 = A + Tnj3, ka+fiOTna + ip. Upon consideration of the formulae below, it
quickly becomes clear that the only nonzero summands of « ♦ t;(7) are those of the
form u(<7)v(r), where a and r are one of the six maximal support elements listed
above in (3.1), which implies that the simimands are all positive. For the sake of a
certain degree of completeness of exposition, we list the possible summands in the
case that 7 = A + m/?. The analysis in the other cases is similar.
u*v{X + mp) = u{X)v{ml3) + u{TnP)v{X) + «(A)t;(A + m/?) + u{X + m/3)u(A)
+ u{n)v{X + mp) + u{X + mP)v{fi) + u{t/)v{X + mp) + u{X + m0)v{y)
+ Y, u{X + i/3)v{jP)+ Y^ u{i/3)v{X + JP)+ Yi u{X + iP)v{X + jP)
i+j=Tn i+j=Tn i+j=m
ijjtO «J?tO ijjto
To compute u * v{X + mP), consider all the possible combinations of the
following situations and then add.
56
1. If «(A) / then if v{mP) / 0, we have that ml3 < /x„. If m/3 < //„, then
7 = A + m/? < A„ + /i„, which is nonsense. Thus m/3 = /i„, in this case and
u{Xv)v(nv) is a summand of it * z; (7).
2. By an argument similar to the above, if u(A) ^ and u(m^) 7«^ 0, then
mP = /iu and M(/x„)t;(A„) is a summand.
3. If u(A) / 0, w(/i) 7^ or u{v) ^ then if t;(A + mp) y^ 0, we may conclude
that 6^ = X + mp and tt(A„)t;((J„),u(/x„)t;((5„) or, respectively, it((J„)u((J„) is a
summand.
4. Similarly, if t;(A) / 0, u(/i) ^ 0, or v{i/) ^ then (J« = A + m/3 and u((J„)u(A„),
m((Ju)w(/A;), or u{6y,)v{5„) is a summand.
5. If, for some i + j = m, we have that w(A + iP)v{jP) ^ 0, then we also have
6^ = X + iP,fiv= JP and u(5„)u(/i„) is the only nonzero contribution from this
large sum.
6. Likewise, iiu{ip)v{X+jP) ^ for some i+j = m, then u{n^)v{6y) is the only
nonzero component of this summation.
7. Finally, if u(X+iP)v{X+jP) / then u{6u)v{6y) is the only nonzero summand
coming from this summation.
As stated before, the analysis in the other two cases is similar. We provide
the formulae below:
u * v{ka + /i) = u{ka)v{iJ,) + u{iJ,)v{ka) + u{X)v{ka + fi) + u{ka + /i)u(A)
+ u{ij)v{ka + /x) + u{ka + fji>)v{fi) + u{t/)v{ka + /j) + u{ka + /i)u{i/)
+ tJ u{ia + iJ,)v{ja) + 22 u{ia)v{ja + n)+ >J u{ia + fx)v{ja + n)
i+j=k i+j=k i+j=k
iJjtO «j5tO ijjto
■■\ y '1 \
t :i.. , :.  OU; 57
>
w * i;(na + Ip) = u{na)v{ll3) + u{l/3)v{na) + u{X)v{na + 10) + u{na + l0)v{X)
+ u{n)v{na + //?) + «(7iQ! + W)v{fj,) + u{u)v{na + 10) + u{na + I0)v{v)
+ u{na)v{\ + //?) + u(A + I0)v{na) + u(na + Ai)u(i/?) + w(f/?)u(na + /x)
+ ^ «(zq; + //?)w(JQ!) + ^ u{ia)v{ja + l(5)\ ^ u{na + i0)v{j 0)
i+3=n i+j=n i+i=i
tj#0 ij^O tj?tO
+ ^ u{i0)v{na + jj5) + ^ M(iQ: + n)v{ja + 10)+ ]^ «(iQ: + I0)v{ja + //)
i+j=J i+i=n «+j=Ti
JjVO iji^Q ij^
+ Y^u{X + ip)v{na + jP) + J^ ^(nQ; + i0)v{X + j/?)
+ Y^ it(za + A;/3)u(ja + m/?)
"R, is not an /ring: Let ^a„+^ = {u 6 7^ : J € Aq+^ =^ v(5) = 0} be the
minimal prime subgroup. Then Xa, X/J ^ ■f^A„+3, yet Xa *X/9 = Xa+/3 ^ f^A„+^. Thus,
^Aa+/3 is not an ideal and 71 is not an /ring by [BKW, 9.1.2]. D
The following two propositions record some consequences of the ^system
condition. Note that none of the excluded conditions occur in Example 3.2.6.
Proposition 3.2.7. Let (A, +) he a bounded above (.system in which maximal ele
ments act as an additive identity on elements below it. Then the following can not
occur for nonmaximal elements a, /? G A,
1. P \ a < a, where 0, a are below the same maximal element, and \\ { a
2. 0<a<0\a
3. p<0la<a
4 If X, n € A are maximal and (A, /x) e A, then X \ fj, it X, fx.
58
In particular, the second and third properties imply that if a, 13, p+a are comparable,
then fi + a <a,p.
Proof: U 13 + a < a, 13 \\ P + a, and ii> a, pis maximal, then xp, X/*  Xa are
positive. Yet, x^ * iXn  Xa) = Xff  Xff+a is negative, contradicting that 7e is an
£ring.
Assume that /? < a < /? + a. Let /x > ;5 + a be maximal. Then as above,
X/3 * iXn ~ Xa) < 0 The same contradiction is obtained in the case that we assume
P<l3 + a<a<n.
If A,/iaremaximaland A + /i </i thenxA*(X/t2xA+;i) = Xa+m < 0 ■
Proposition 3.2.8. Let (A, +) be an isystem.
1. There is no nonmaximal 5 G A such that 5 is idempotent and 6 + a = 6 for
all nonmaximal a> 5.
2. If 6 E A is nonmaximal and idempotent then there does not exist a maximal
element ijl> 6.
Proof: If (5 e A such that 6 is idempotent and 5 + a = S for all nonmaximal a> 5,
Then we contradict the assumption that 71 is an ^ring since xs * (Xq ~ ^Xs) — —Xs
Ji fi> 6 is maximal and 6 is idempotent, then xs * (X/i — ^Xs) = —Xs ■
3.3 fSvstems
We are much more interested in the /ring situation. In [CMc, 2.1], the
authors demonstrate a condition on A which will give rise to an /ring.
Theorem 3.3.1. Let (A, +) be an (.system. Then It is an fring if and only if the
root system also satisfies: i/a  /? and {a,^) e A, then a + 'y \\ 13; and 1/(7, a) G A,
then 7 I a II y3.
59
In this section we consider a condition on A which is equivalent to the one
stated above and proceed to investigate certain properties of the associated /rings.
Recall that r(F(A,R)) is orderisomorphic to A by Proposition 2.1.1, where the
values are of the following form, for 6 E A:
Vs = {uen:'y>S=^ «(7) = 0}.
Definition 3.3.2. Let (A, +) be an isystem such that 5 = a + /3 implies a,j3>5.
Then we say that (A, +) is an /"system.
Proposition 3.3.3. // (A, +) is an f^ system, then % is an iring and for every
(J G A, the subgroup Vg is an ideal. Hence, in particular, 71 is an fring and 3? is
an fsubring of H.
Proof: Assume that (A, +) is an /"system. Let 6 € A,v eVs^ueH, and j >S.
Let 7 = a+(3, then by the /"system assumption, ^ >'y>6 and hence v{P) = 0, for
aJl such P since w G V^. Then u*v{'y) = X)a+/9=7 u{a)v{P) = and we conclude that
u*v eVs Therefore, the values oCR. are ideals. Moreover, since eadi minimal prime
subgroup is an intersection of a chain of values by [D,10.8], each is an intersection
of a chain of ideals; thus each minimal prime subgroup itself is an ideal. Finally, we
have shown that 7e and 3? are /rings by [BKW, 9.1.2]. ■
Proposition 3.3.4. An tsystem (A, I) is an f^ system if and only if K is an
fring.
Proof: Sufficiency is shown in Proposition 3.3.3. Conversely, assume that there
exist a,/3 E A such that af/?^aorQ;^^/3. Then by Proposition 3.2.7, we
have that a, /3, a + /9 are not all comparable. If a \\ /?, then we have that Q\\a\ (3
by Theorem 3.3.1. Assume that 13 \\ a\ P and let C be a maximal chain in A
containing a\ pLei Hc = {v €%: 5 EC ^ v{8) = {}i}\ie the associated minimal
60
prime subgroup. Then X0 ^ He, yet Xq * X/9 = Xa+ff i He Thus He is not an ideal
and hence Tl is not an /ring by [BKW,9.1.2]. ■
Definition 3.3.5. An f^system satisfying the following is called an /system;
l.Ifa and {3 are comparable, then {a,P), {P,q) G A.
2. If IX is maximal, then {S, /x), (/x, 6), {n, fi) € A and 5 + n = ii + 6 = Sfor every
6 < fi. In particular, /x I // = ju.
We will short;ly see that these additional assumptions on a bounded above /°
system make maximal chains in A into monoids. This is quite useful in our setting.
For instance, it is not difficult to figure out when the /rings have a multiplicative
identity, under the /system assumption.
Proposition 3.3.6. Let (A, I) be a bounded above f system. 8? and % each have
a twosided multiplicative identity if and only if A has a finite number of connected
components.
Proof: Let (A, +) contain only a finite number of connected components with
maximal elements {//i, /i2, •■ • , Mn} Let Xj be the characteristic function on the set
{fij} and let e = Y,%i Xj Then for w 6 71 and 5 G A,
V * e{6) = ^ v{a)e{f3) = v{6)e{fjtk) = v{6),
where /Lt* > (J is maximal. Likewise,
e*v{6)= Y^ e{a)v{0) = e{iJik)v{6) = v{6).
Thus e is a twosided multiplicative identity.
Conversely, assume that e G 7^ is an identity and let /x G A be maximal with
characteristic function Xn Then 1 = X/iCa*) = e* Xm(/^) = e(/i). Thus, supp(e) has
* ST
61
as many maximal elements as there are maximal elements in A. Since e G 72., we
must conclude that there are only finitely many connected components in A. ■
It is handy to have the following definition:
Definition 3.3.7. Call an rsystem (A, +) unital if the ring % has a multiplicative
identity. .^  • ^
The rings 3? and "R, are semiprime, as we will now show. Thus the minimal
prime subgroups are also prime ideals, by [BKW, 9.3.1].
Proposition 3.3.8. Let (A, +) be an f^ system. Then K is semiprime.
Proof: Let u e 11, where u = Y^j^iCtjXSjj for an index set / of supp(«) and for
ttj e R\0 for all j 6 /. Then u*u{6) = Y^s=s.^s ^»%" ^^^ '^« ^® maximal in supp(u).
We show that Si + Si is maximal in supp(u * u). Let S = Sj + Sk ^ supp(u * u).
If Sj II Si, then by the /"system condition, 5  (Jj + Si. On the other hand, if
Sj, Sk < Si, then S < Si + Si,as desired. Thus u * u{Si + Si) = af > and we conclude
that Tl is semiprime. ■
Corollary 3.3.9. Let (A, +) be an f^ system. Then maximal chains in A are closed
under the operation +.
Proof: Let C C A be a maximal chain and let He — {v € H : S e C =^ v(S) = 0}
be the associated minimal prime subgroup. Let a,/? e C and assume by way of
contradiction that a + /? ^ C. Then Xa*Xfi — Xa+fi € He but Xa,X$ ^ He Thus
He is not a prime ideal. This contradicts Proposition 3.3.8, by [BKW, 9.3.1]. ■
One should ask if the maximal ^ideals of TZ are actually the maximal ideals;
or equivalently, one should ask if "R has the bounded inversion property. The answer
is yes, if (A, +) is a unital /system.
62
Proposition 3.3.10. Let (A, +) 6e a bounded above unitd f system. An element
ueU is a multiplicative unit if and only ifu^Vf^ for all maximal /i e A. Thus 71
satisfies the bounded inversion property.
Proof: Let 5" C A be the set of maximal elements of A and let e = Zl^es Xit be
the multiphcative identity. If u e 72. is a multiplicative unit, then there exists v eK
such that u*v = e. Hence 1 = e()u) = u{fjL)v{fj.) for every /x 6 5". Thus «(//) / for
all // G 5 and hence u ^ V^ for all maximal /x e A, as desired.
Assume that u ^ V^ for all maximal /i e A. We define the multiplicative
inverse t; of u recursively on each maximal chain in the support of u. Let v{5) =
for all 6 ^ supp(m) and let t;(/x) = 1/m(/x) for each maximal /x € A. Then ifSi<n is
maximal in supp(n), we just solve the equation = u*v{6i) = u{Si)v{iJ,) + u{fi)v{6i)
to get that v{Si) = u((Ji)/u(/x)^. Proceed with the definition of u accordingly. That
is, let fi> S e supp(u) and assume that i;(7) is defined for all 7 > 5. Then u{a)v{T)
is a summand of u * v{6) only if (5 < a, r e supp(u). Thus v{5) is the only unknown
in and is the unique solution of the equation = u*v{6).
Hence, since if e is the multiplicative identity of 7Z and u > e then u ^ V^
for all maximal /x, we have that u is a multiplicative unit. Therefore K satisfies the
bounded inversion property. ■
Let (L, <) be a partially ordered set. Recall that A C L is called closed if
{ajig/ and AjCj or VjCi exists in L then AjOj, Vjaj e A. It is the case that, if (A, +)
is an /system, then all of the closed convex ^subgroups of 7^ are ring ideals. We
will use the following special case of [D, 45.26].
Theorem 3.3.11. Let G be a finitevalued £ group. Then there is an orderpreserving
correspondence between the closed convex isubgroups of G and (order) ideals $ of
the root system T{G) given by
K t^^K = {Gs e r{G) •.3k e K such that Gs is a value of k}
63
^ i^ K^ = {g € G : all values of g are in $}.
Proposition 3.3.12. Let (A, +) be a bounded above f system. Any closed convex
isubgroup of TZ is an ideal of 7Z.
Proof: Let A' be a closed convex ^subgroup of H; let it G /('+ and t; G 72.. If u is
a unit, then $k = r(G) and by the preceding theorem, K = G. So assume that u
is a nonunit.
Let /x G A be maximal and assume that u * v{'y) = for all /x > 7 > 5 and
u*v{5) ^ for some 6. By the preceding theorem, we need to show that S G $« C A.
Let 6 = a\ 13 such that u{a)v{P) ^ 0. Then there is a maximal a' > a such that
u(a') ^ 0. Then a' = a' + fj, > a' + p > a + p = 5 md a' E ^k since u e K.
Therefore, we conclude that 6 G ^k since ^k is an order ideal. Hence, u*veK
by the preceding theorem. ■
We now seek the prime ^ideals and 2ideals among the prime subgroups Vg
and their associated value covers. The cover of Vs is the set
Ps = {ven: v{j) = for all j > 5}
and is the smallest convex ^subgroup properly containing Vs and xs First let us
recall the most general definition of 2;ideal. Let G be a vector lattice, v G G"*",
and let {^n}^i C G be a sequence. Recall that the sequence converges relatively
uniformly to g €. G along the regulator v and write gn — > g, if for every e >
there exists iVe > such that for all n > iV^, we have that [^ — ^nl < ^v Let
H he a, convex ^subgroup (subvector lattice) of G. The pseudoclosure of H is
H' = {g € G : ^gn}n=i ^ H,gn ^ g for some v G H+}. Then H is relatively
uniformly closed ii H — H'; let H denote the relative uniform closure of H. Then
if G{g) denotes the convex ^subgroup (subvector lattice) of G generated by g,
we define a convex ^subgroup (subvector lattice) if to be an abstract zideal if
64
he H,g eG md G{g) = G{h) imply that g e H.ln fa^t, [HudPI, 3.4] says that
this definition is equivalent to G{h) C H for all he H.
Proposition 3.3.13. Let (A,+) be an f system. If fi G A is maximal, then V^ is
a prime ideal of H which is an abstract z ideal.
Proof: Let u * v €. V),, and u ^ V^. Assume without loss of generality that
w+jV"^ € Vft and u~ ^ V^. We show that v~ £ V^. Since u~ ^ V^, we have that
«"(/i) ^ 0. Thus since u"*", V^ eVft,
a+fi=tt a+0=ft
and since /i is maximal, we have that = u~(/i)u~(/i) and therefore, v~{n) = 0,
This gives that t;~ e V^ and u € V^. Hence V^ is a prime ideal.
Since Tl/V^ is isomorphic to R via the evaluation map u i> u(/i), we have
that V^ is uniformly closed by [HudPI, 2.1]. Thus by [HudPI, 3.4], V^ is an abstract
2;ideal. ■
Corollary 3.3.14. 7f (A, +) is a bounded above unital f system, then the maximal
ideals are given by {V^ : // 6 A is maximal}.
Proposition 3.3.15. Let (A, +) be an f system and let 6 e A be nonmaximal
Define Ps = {v €1I: v{'y) = for all 7 > 5}. Then Ps is an abstract zideal and
it is a prime ideal if and only ifa^j3>6 for dla,^ > S.
Proof: Let v € Pg. Then there exists {u„}^i C Ps such that t;„ ^ v, for some
w G TZ."*". Let 7 > (J, then for every e > we have that u(7) < 610(7). Thus ^(7) =
and Ps is relatively uniformly closed. Therefore, Ps is a 2ideal by [HudPI, 3.4].
Assume that Ps is a prime ideal and that there exist a,0 > 6 such that
a + 13 < 6. Then Xa+0 G Ps and Xa+0 = Xa * Xp But Xa.Xp i Ps, which is a
contradiction.
Conversely, assume that a+ P > 5 for all nonmaximal a,/3 > S. Let u*v £ Ps
and assume, by way of contradiction, that u,v ^ Ps Then u * v{^) = for all
7 > (J and there exist elements a e supp(u) and ^ e supp(t;), each maximal in the
support set and such that a,P > S. Assume, without loss of generality that a > /?. If
o^ <a,iy < and at least one of the inequalities is strict, then a' + /?' < a + ^ and
we conclude that u*v(a + l3) = u{a)v{l3) + u{f3)v{a). If a = /?, then since a + /3 > 5,
we have that = u*v{a + /3) = 2u(a)v{/3) ^ 0, a contradiction. If a > /3, then
v{a) = and hence = u* v{a + P) = u(a)v{P) / 0, another contradiction. Thus
we conclude that either u or t; is in Ps and therefore Ps is a prime ideal. ■
Let 5 be a totallyordered set. Recall that S is an rjiset if whenever A,B C S
are countable and A < B, then there exists c G 5 such that A < c < B. Since R
is not an 771 set, the ring H is never an 771 set. To see this, let 5 G A and consider
the sets {xs} > {(1  ^)xs '■ n G N}. But, H can satisfy a related, slightly weaker
condition.
Definition 3.3.16. We call a totally ordered set S an almost rjiset if A,B C S
are countable and if A < B, then there exists cE S such that A<c<B. Note that
E is such a set.
Proposition 3.3.17. Let (A, h) be a totally ordered f^ system. H is an almost
Tjiset if and only if A is an rjiset.
Proof: Assume that 71 is an almost 771set. We first note that A contains no suc
cessor pair. Let a < /? be a successor pair. Then the sets {nxa}n&i and {l/nX/3}n6N
contradict the almost r/iset condition.
Let A = {Q!j}jeN,B = {Pj}jeJi C A, where ai < a2 < ■■■ < Ih < Pi Then
Xai < Xa2 < • • • < X/?2 < Xffi and there exists f ell such that x<m < f < XPj for
all i,j. Let 7 be the maximal element of supp(/). Then f — Xai > implies that
7 = ttj and /(7) > 1 or 7 > a^ and 7(7) > 0. If 7 = a^, then /  Xa+i < 0, which
66
is a contradiction. Thus 7 > a^ for all i. Similarly, 7 < /?, for all j. Therefore, A is
an r^set.
Conversely, assume that A is an r7iset and let /i < /2 < • • • < 52 < 5i ^ ^•
Let ^1 < 02 < * * • < 72 < 7i ^ A be the corresponding maximal elements of the
support sets. Let $ = {(j>j}jefi a^^d T = {7j}ieN We have a few cases to consider: $
and r are the same constant sequence, one of the sequences is eventually constant,
or neither sequence is eventually constant.
If there exists n € N such that (ffj = a = ji for i,j > n, then we get the
following string of inequalities in R : /i(0i) < f2{<h) < '• < 52(72) < 5i(7i) Since
R is an almost r/iset, there is r G R such that fj{^j) <r < Qiiji) for all i,j. Then
fj < fXa < 9% for all i, j. If $ is eventually constant and F is not, say (f>j = a for
all i > n, for some n, then, by hypothesis, there exists /? e A such that a < /? < F.
Then /_, <Xp^9i for a,ll i, j. If neither sequence is eventually constant, then by the
t/iset hypothesis, there exists ^ such that 0j < ^ < 7^ for all i and fj < Xff < 9i for
all«,j. ■
3.4 Survaluation Ring and n*^Root Closed Conditions
Recall that a commutative ring ^4 is a survaluation ring (or SVring) if A/P
is a valuation ring for every prime ideal P. In this section, we set down a character
ization of those /systems which give SVrings.
Let (A, <,(, /i) be a totally ordered and cancellative abelian monoid with
identity element fx. We define the group of differences, qA, of A as it is done in
[Fu, X.4]. Define an equivalence relation on A x A by (61,52) ~ (71,72) if and only
if Si + j2 = ji + S2. It is clear that the relation is reflexive and symmetric; the
transitivity follows from the cancellation property of the monoid. We let qA be the
quotient A x A/ ~, denote the class of the element (61,62) by [61  62], and define
an operation + as is usual. That is, [61  62] + [71  72] = [(61 + 71)  ((J2 + 72)] The
67
cancellation in the monoid ensures that the operation is welldefined. The element
[^  /i] is an identity and [^2 — (Ji] is an additive inverse of [5i — S2]. We define
[Si  82] < [71  72] if and only if (Ji + 72 < 7i + 82. By [Fu, X.4.4], this is the
unique order on gA which extends the order on A. Finally, oembed A in 9 A via
Let ff be a partially ordered groupoid. Then h € H is called negative if
hx < X or xh < x, or both for all x e H. The groupoid H is called negatively
ordered if every element is negative.
Definition 3.4.1. Let A be a partially ordered semigroup. A is called inversely
naturally ordered 1/ it is negatively ordered and 6 < a implies that there exists
/? € A such that (J = a + /?.
Example 3.4.2. Let A = {1  ^}^i U [l,oo) C R be inversely ordered with the
usual addition in the reals. Then A is an /system which is not inversely naturally
ordered. To see this, note that (1 — ^) + c = 1 if and only if c = ,ti = 2 or
c=l,n=l. D
Let (A, f, //) be a totallyordered abelian cancellative monoid with maximal
element /i, such that (A, I) is an /system. Let X = {x* : <J € A}. Then (A", *,Xii)
is a totallyordered abelian monoid which is ^isomorphic to (A, ,/i). Since X is
written multiplicatively, the elements of the group qX are quotients and we denote
them as such in the following proof.
Theorem 3.4.3. Let (A, +, fx) be a totally ordered abelian cancellative monoid with
maximal element fx. Let X = {xs : 6 e A} and TZ = F(A,R). The following are
equivalent:
1. TZ is a valuation ring.
r; '; ■ . ■ . "i r 1^*'^":,.)
2. % is 1convex.
68
S. His Bezout.
4. V, is convex in qH.
5. X is convex in qX.
6. A is convex in qA.
7. A is inversely naturally ordered.
Proof: The equivalence of (1), (2), and (3) is [MW, Theorem 1]; the equivalence
of (2) and (4) is [ChDi2, Lemma 2].
(2) =i> (5) : Let X7 < xjxp < Xs Then < %„ < Xs+fi and by (2), there exists
fen such that Xa = f * Xs+p ^ f = JLjejfi^<t>p ^^^^^ "^ ^ ^^^ ^°^®^ ^®*'
fj e R and 0j G A for all j e J, then
ifj if'
[0 els
Thus, for some j G J, we have that a = /? + <J + (^j, /j = 1 and fk = Q for all
A; 7^ j. Hence / = x^^ and Xa/X/3 = X(J+*, £ A", as desired.
(5) <=> (6) : Follows since A^X.
(5) =^ (7) : Let a < ^ e A. Then a + ^ < a and therefore Xa+/3 < Xa < Xi9 which
implies that Xa < Xa/Xfi < X/x Thus, by (5), we have that Xa/Xfi = X6 ^ X for
some 6 e A. Therefore, Xa = X^+s and a = f3 + S.
(7) => (2) : Let < u < v £ TZ, and assume that u = YljeJ^J^"'} ^^'^ *^^*
V = Y!,k£K ^kX0k fo^ index sets J, iif, and where Oj, 6^ G R for all j G J, A; G iiT.
Also assume that ai < Pi are maximal elements in the respective support sets.
Then by (7), for every j G J and every k e K, there exists 6j,jk G A such that
Qj = Pi+ Sj and pk = pi+ 7fc. Note that 71 = //. Then u = X/9i * Z)jej OjX*, and
69
Let w = Ylk^K^kXik ^^^^ w{n) = bi > and hence ^w is invertible in
U. Let X = Y^j^j ttjXSj ■ Then by cancellation in A, we have that < x < u; and
< ^x < ^w. If we let / = (^w)~^ * (^x), then ^x = j^w * f ajad x = w * f.
Finally, it = X/3i * x = X;8i * i« * / = u * /, as desired. ■
The following lemma is wellknown and routine to verify.
Lemma 3.4.4. Let (A, +) be an (.system andC C A, a maximal chain. Denote the
associated minimal prime subgroup by He = {u ^ Tl : C Ci supp(«) = 0}. The map
(p : F(A, R) ^ F{C, R) given by restriction is a surjective iring homomorphism
mth kernel He. Thus, n/He = F{C, R)
Corollary 3.4.5. Let (A, +) be a unital f system. Then Ti is an SVring if and
only if each maximal chain in A is inversely naturally ordered.
Example 3.4.6. 1. If Ai = [0, oo) C R is inversely ordered with the usual
addition of real numbers, then F(Ai,R) is an SVring.
2. If A2 = {1 — }^i U [1,00) C R is inversely ordered with the usual addition
in the reals, then F(A2,R) is not an SVring.
3. Let A = R[[x, y]] be the ring of formal power series in the indeterminates x, y.
Order the monomials lexicographically via 1 » x, y and xV < x*y' if and
only if A; < z or A; = i and I < j. The ring A is not an SVring since it is
not 1convex: note that < y < x and the equation y = xf has no solution
f eA.   V . . , ... ' .'
Let Zx = Zj, = {n e Z : n > 0} be inversely ordered. In the lexorder
described above, if A = Z^ x Zy, then A = F(A, R). We convexify A in qA by
convexifying A in qA. That is, if A'' = A U {(n,m) e Z x Z : n > 0,Tn < 0}
then ^(A^ R) ^ A{{xY : i > or i = and j > 0}) U A is an SVring which
is the convexification of A in qA. D
70
Recall that an /ring A is n^root closed if for every a ^ A^ there exists
h ^ A such that a = 6". This property arises in K if there is a certain amount of
divisibility in the arithmetic structure of A.
Definition 3.4.7. Let (A, +) he an r system. A is coiled 7idivisible if for every
6 e A there exists a G A such that na — 5. We say that the system is divisible if it
is ndivisible for all n EN.
Proposition 3.4.8. Let (A, +) be a totally ordered f system such that 71 is n^^root
closed. Then A is ndivisible.
Proof: Let 5 G A. Then xs = w" for some v = Ylj^jO'jXaj li S = fj. is maximal
in A, then 5 = nfx, so we assume that (J / /x. If ai € supp(t;) is maximal in the
support set, then nai e supp(t;") is maximal. Therefore 6 = nai, as desired. ■
Proposition 3.4.9. Let (A, +) be a totally ordered inversely naturally ordered f
system with maximal element fi. If A is ndivisible, then 72. is n^root closed.
Proof: We begin with squareroots. Let u G TV^ be given by u = X^.^j 0,3X0, for
some index set J, Uj G R, and «_, 6 A for all j G J.
K u{ii) ^ 0, then we define a squareroot v recursively on A. To begin, let
v{n) = yju{n) and assume that ai is maximal in supp(u) \ {/x}. Let v{S) — Q for
all ai < (J < /i. Then we define v{ai) = u{ai)/2v{n). Let S be the Nlinear span
of supp(u) and define u((5) = for alH ^ 5. If ^(7) is defined for all 7 > (J, then
we define v{5) to be the unique solution of the equation u{5) = ]C(r+T=<5^(^)^ (''")'
where, necessarily, a,T E {a e S : a> 6}. Then u — v*v,hy construction.
Now, assume that u(/i) = and let ai be the maximal element in the support
of u. Since A is inversely naturally ordered, for every j 6 J, there exists Sj e A such
that aj = tti ^Sj. Note that 61 = //. Then u = Xai *T>jeJ ^jXSj • Let w = "^.^j ajxsj ,
then w{ij,) ^ 0. Thus w = vi*vihy the above construction. Since A is 2divisible,
71
ai = 271 for some 71 £ A. Therefore Xa = Xn * Xn ■ Letting u = X71 * ^i' ^e then
have that v*v = Xyi*Vi*X'n*'Vi= Xai*w = u. Thus n*''roots exist when n is a
power of 2.
Let n be odd and let w € 71 be given by t; = X)fc€Ar ^^X^k ^^^ some index
set K, bk € R, and /?*; € A for 8ll k e K. As with square roots we consider two
cases. First assume that v{fi) =^0 and define an n^'root recursively. Let w{fj,) be a
real n*''root of u(/x) and let v{6) =0 for all ^i < 6 < fi, where pi is the maximal
element of supp(t;) \ {/i}. Then the Tvfold convolution product equation we must
solve reduces to v{Pi) = w"(/?i) = n{w{n))^~^w{f3i). In order to see this, we proceed
by induction on n. If n = 2, then w*w{Pi) = w{tj)w{Pi)+w{Pi)w{fi) = 2w{fj,)w{l3i).
Assume that the statement holds for n = m. Then
t/;'"+^(A) = {w * w"'){/3i) = w{i^)w"'iPi) + t/;(A)ti;"'(/i)
= wifi)miw{fM)r'w{l3,) + «;(A)(w(/i))'" = (m + l)(«;(//))"*w;(/3i)
as desired. We may then define w(/?i) = ^(/^i)/("(^(m))''~^)
In general, we let ^(7) = if 7 is not in the Nlinear span of the support of v.
Assume that 10(7) is defined for all 7 > (J. We show that the equation v{S) = w^{5) is
linear in 'w{5); hence, we may define w{6) to be the unique solution of this equation.
If n = 2, then w'^{S) = Y!,a+T^s w{a)w{T) + 2w{5)w{fj,). Assume that w^{6) is linear
in w{6). Then
«,"*+! (,J) = ^ w{(t)w"'{t) + w{jji)w"'i6) + w{5)w"'{fi)
ff+T=S j
is linear in w{6), by induction, since w{5) will not appear in w"*{t), as S <t and A
is an /system. Thus, in this case, v has an n*''root.
Second, assume that v{fi) =0 and proceed as in the squareroot case. Let /?i
be the maximal element in the support of v. Since A is inversely naturally ordered,
for every k £ K, there exists 5k £ A such that fik = Pi + Sk Note that 61 = //. Then
72
^ = X/J, * 'ZkeKhXs, Let x = Y^keK^Xs,, then x{n) ^ 0. Thus x = w^, for some
wi, by the above construction. Since A is ndivisible, /?i = n7i for some 71 e A.
Therefore X/9, = (Xti)" Let to = X7, * Wi; then lu" = (X7, * Wi)" = X)3, * a; = t^. ■
Corollary 3.4.10. Let (A, +) be a totally ordered inversely naturally ordered f
system with maximal element fx. A is divisible if and only ifKis n^^root closed for
alln.
Example 3.4.11. 1. Let Ai = [0, 00) C R be inversely ordered with the usual
addition of the real numbers. Then F(Ai,R) is n*''root closed for all n.
2. Let A2 = {n € Z : n > 0} be inversely ordered. Then A2 is not 2divisible
and Xi > has no squareroot. That is, if Xi = v^, then v{0) = and we then
conclude that 1 = Xi(l) = 2v{0)v{l) = 0, a contradiction.
3. Let A3 = {1  }^3 U [1, 00) C R be inversely ordered with the usual addition
in the reals, then A3 is not inversely naturally ordered and similarly, xi has
no squareroot since 1 has no nonzero summand. D
Proposition 3.4.12. Let (A, I) be a totally ordered f system with maximal ele
ment fi. Then if n has an immediate predecessor tt, then Xv has no squareroot. If
there exists /i > (J G A such that 6 has no nonmaximal summand other than itself,
then xs has no squareroot.
Based on the preceding examples and results on n*''roots, we formulate the
following:
Conjecture 3.4.13. Let (A, I) be a totally ordered f system with maximal element
/i. F(A,R) is squareroot closed if and only if A is 2divisible and every nonmaximal
element of A has a nonmaximal summand other than itself.
73
Recall that a field K is realclosed if every positive element is a square and
every polynomial p € K[x] of odd degree has a root in K. An integral domain
R is called realclosed if qR is a realclosed field. Let (A,) be a totally ordered
inversely naturally ordered /system with maximal element fj,. Assume that A is
also 2divisible. Then 7^ is a 1convex and squareroot closed /domain. By [ChDil,
Theorem 1], under these conditions, 72. is realclosed if and only if every odd degree
polynomial over H has a root. What additional assumptions on A are necessary to
guarantee the realclosed property?
Conjecture 3.4.14. Let (A,) be a totally ordered commutative f system with
maximal element //. // A is divisible and inversely naturally ordered, then TZ is real
closed.
We end this section by shedding a little light on this conjecture. Recall from
[HLM] that a commutative /ring A with 1 satisfies the Intermediate Value Theorem
for polynomials (or is an IVTring, for short), if for every p{t) 6 A[t], and pair of
distinct elements u,v E A such that p{u) > and p{v) < 0, there exists w € A such
that p{w) = and uAv<w<uWv. We show that a totally ordered commutative
semiprime valuation /domain with identity is realclosed if and only if it is an IVT
ring. It is necessary to record the following unpublished theorem of Suzanne Larson,
which was communicated via electronic mail on April 17, 1997. Her proof follows.
Theorem 3.4.15. Let A be a commutative semiprime IVTring tvith identity. If
S is a multiplicatively closed subset of regular elements of A'^, then the ring of
quotients, S~^A is an IVTring.
Proof: Let p{t) e S'^A[t] be given by p{t) = oo6o ^ + aiK^t H h a„64" and
assume that p(itii;f ^) > and p(u2U^^) < 0. Let d = v^v^bobi • • •&„. Then d e 5 is
regular. Define a new polynomial q{t) e A[t] by
oov^v^bi • . • bn+aiv^^v^%b2 • • • 6„<+02<"M~%&i^ • ■ • bnt'^+ • •+anbobi • ■ • 6„_ir.
74
Then q{uiV2) = dp(uit;f ^) > and q{u2Vi) = dp(u2U^^) < 0. Since A is an IVTring,
there exists w E A such that uiV2 A U2V1 < w < U1V2 V U2V1 and q{w) = 0. Then
UiVi^ Au2V2^ < wVi^V2^ < uiVi^ Vu2^^j\ and dp{wv^^V2^) = q{w) = 0. Since d is
regular, p{wVy^V2^) = and we conclude that the quotient ring is an IVTring. ■
Proposition 3.4.16. Let A be a totally ordered commutative semiprime valuation
f domain with identity. Then A is realclosed if and only if it is an IVTring.
Proof: If A is realclosed, then qA is a realclosed field and, by [ChDi2], qA is an
IVTfield. Let p{t) G A[t] be such that p{u) > and p{v) < 0, for some u,v E A.
Then there exists w e qA such that p{w) = and u Av < w < uV v. Since A
is a valuation domain, A is convex in qA by [ChDi2, Lemma 2]. Hence, w E A
and we have that A is an IVTring. Conversely, if >1 is an IVTring, then qA is an
rVTfield, by the preceding theorem of Larson. Then, by [ChDi2], qA is realclosed
and therefore A is realclosed. ■
t.' ' .' \
♦
CHAPTER 4
RAMIFIED PRIME IDEALS
In this chapter we expand on the notion of a ramified prime ideal, as defined in [Le],
which we introduced in Chapter 2. We first examine the concept in general and then
move to try to understand ramified maximal ideals which correspond to nonisolated
G^points. This result is then used to consider local versus global ramification
conditions.
4.1 Ramified Points
Definition 4.1.1. Let X be a completely regular space. A prime ideal ofC{X) is
ramified if it is the sum of the minimal prime ideals that it contains. We define
p £ X to be ramified if Mp is ramified. A point p G X is totally ramified if every
prime zideal contained in Mp is ramified. The space X is (totally) ramified if every
nonisolated point of X is (totally) ramified.
A ramified ^ideal of C{X) is a prime ideal, by Corollary 2.2.6. LeDonne
proves that a ramified prime ideal of C{X) is necessarily a 2;ideal. Let us consider
two extreme conditions. Recall that we say a point p G X is an Fpoint if Op is
prime. If p is an Fpoint, then since Op is the unique minimal prime ideal contained
in Mp, Mp is not ramified. Likewise, in this case, no prime 2ideal contained in Mp is
ramified. On the other hand, the condition of total ramification ensures branching
at every prime zideal. Aneilytically, LeDonne shows [Le,§ 3]:
Theorem 4.1.2. IfX is a metric space then every maximal ideal ofC{X) is totally
ramified.
75
76
Note that this result says that every maximal ideal of C{X) (fixed or free) is
ramified, if A" is metric. We do not know of any weaker topological condition which
guarantees total ramification of C{X).
Definition 4.1.3. Let A be a commutative fring with identity. For any integer
n>2, call a prime £ideal P nlimbed if P is the sum of n noncomparable prime
iideals of A which are properly contained in P. A point p of X is nlimbed if Mp is
nlimbed. Note that any nlimbed iideal P is necessarily ramified and rk(P) > n.
Example 4.1.4. We now present an example of an /ring 11 in which a maximal
ideal is ramified but not Tvlimbed for any n. Let A© = [0, oo) C R and define
Ajj = (l/n,oo) C R, for i = 1,2, where each interval is inversely ordered. Let
A„ = A^ U A^ be the disjoint union and then let A = A^o U (n„ea,An). We obtain a
root system with the induced ordering which we describe in the following diagram:
(4.1)
We endow A with a partial commutative associative binary operation. Let
(*)^ denote the sum in parentheses as the usual sum of real numbers residing in Ajj;
77
the mark "" signifies that the sum is undefined. Note, to conserve space, the table
is completed by reflection across the diagonal.
+
re Ao
re a;.
reAi.
reA^
reAf
»€ Ao
(r + «)o
ir + sy„
ir + s)l„
ir + sfi
ir + s)^
s&K
(r + s);
(r + «)A
(r + 3)i if fc < n; else 
(r + s)f if J < n; else 
»eAi.
(r + 3)j„
(r + a)l if A; < m; else 
(r + s)f if J < m; else 
sG A^
'(r + s)l

seA?
(r + »)f
Let n = F{A,R) and I>„ = Ao U (U^>„A;„); let Co = Ao U (IIn6u,A;,) and
C„ = Pn II A^. Then the minimal prime ideals of H correspond to these maximal
chains and are given by Q„ = {u G 71 : u{Cn) = {0}}, where n = 0,1,2, —
Any supreraum \/j^jQj over a finite set J C a; is the prime ideal P^ given by
{u € 71 : u(I>m) = {0}}) where m is the maximum element of J. Hence, for all
new, the maximeil ideal Vo is not nlimbed since it is not a finite supremum of
minimal prime ideals. However, Vq = Vnew^"' ^^^ ^ ^* ^^ ramified. D
We show in Proposition 2.3.4 that for any nonisolated point p of a first
countable space, the maximal ideal Mp is nlimbed for every n. If the space is also
cozerocomplemented, then rk(Mp) > 2* and Mp is 2*limbed. Prom this, we also
obtain the following, which is weaker than Theorem 4.1.2:
Corollary 4.1.5. Every metric space is ramified.
The following theorem means that if X is a metric space that is not pseudo
compact, then there exist points of ffX \ X such that Af is minimal. Hence, not
every maximal ideal ofC{X) branches nontrivially in the root system Spec(C(X)).
Recall that we call a topological space X perfect if every closed set of X is a G^set.
Note that any metric space is perfect. A point of PX is remote if it is not in the
/?Xclosure of any nowhere dense subset of X. A point p 6 I3X \ A^ is a Cpoint
if p ^ intpx\x{dffxZ{f) r]pX\X) for all / e C{X). A theorem similar to the
following appears in [W, 4.40]. All the proofs there carry through here, verbatim,
under our reduced hypotheses.
78
Theorem 4.1.6. Let X be a completely regular space and consider the following
conditions on a point p G PX \X:
1. p is a C point.
2. Z[M^] contains no nowhere dense set.
S. MP = OP.
4. p is a remote point.
Without additional assumptions, (3) =r' (1). Let X be perfect and assume the exis
tence of a remote point, then (4) => (2) => (3). If X is perfect and the set of isolated
points of X has compact closure in X, then (2) =» (4). If X is realcompact and
C points exist, then (1) =^ (2).
It is not known if a remote point p always has the property that M^ = O^.
We do know the following about the rank of a remote point:
Proposition 4.1.7. Let X be a completely regular space. Let p € pX be a remote
point. Then rkc(x){MP) = 1.
Proof: In [vD, 5.2], it is demonstrated that no remote point is in the closure of two
disjoint open sets of pX. Thus, in particular, no remote point is in the closure of two
disjoint cozerosets of pX. By [HLMW, 3.1], we have 1 = rk^xip) = 'rkc{fix){M*'').
Finally, since C{pX) is rigid in C{X), we have rkc(x){MP) = 1. ■
Finally, we ask: does ramification of a point in X indicate ramification in
PX, or vice versa?
Proposition 4.1.8. A point p £ X is ramified in X if and only if it is ramified in
PX. Likewise, a point p € vX is ramified in vX if and only if it is ramified in pX.
Proof: This is a corollary of Proposition 2.2.12, since we know C{PX) = C*{X)
79
4.2 Ramified G^points
The main theorem, Theorem 4.2.5, of this section provides a good method
for checking the ramification of G^points. We will use it to characterize ramified
Gtfpoints in normal countably tight spaces and to find some ramified points in
product spaces. We first discuss the following proposition, which we will obtain as
a corollary to Theorem 4.2.5.
Proposition 4.2.1. Let p € X be a Gspoint. If X\p is not C* embedded in X,
then rkip) > 2.
Since an Fpoint has rank 1, the preceding proposition, proved in [Le] and
(in greater generality) by van Douwen in [vD], shows that a G^point, p, is not
an Fpoint if it has the property that X \ p is not G*embedded in X. We give a
counterexample for the converse if the G^condition is lifted.
Example 4.2.2. Let X = BaCN, /3N \ N) be as defined in Example 5.2.5. There,
we show that there exists a point p of the corona such that rk{p) = 2, X \p is
G*embedded in X and p is not ramified. But no point of the corona is a Gg. □
The following two results are Theorems 2.1 and 2.2 of [K2]; we will use these
to prove our theorem on the ramification of G^points.
Theorem 4.2.3. Let p be a nonisolated Gspoint of X. If Z € Z[C{X \ p)] then
dx{z) E z[c{x)]. ■" ' ■; " ^ "^
' :■; ■■ ■\'. » ,. i : . > . ; i » ■ :• . „'",
Define 7 : Z[C{X\p)] ^ Z[C{X)] by j{Z) = clx{Z). Let (^ be the
extension of the identity map X \p — y X to the largest subspace Xi C p{X \p)
such that it is extendible as a continuous map into X.
80
Theorem 4.2.4. Letp be a nonisolated Gspoint of X then
1. The mapping 7 is onetoone from the set of prime zfUters on X\p converging
to points o/(A~n{p}) onfo the set of prime zfUters on X contained properly
inZ[Mp].
2. A prime z filter W on X\p converging to a point of(f)~^{{p}) is a zultrafUter
if and only if j{W) is maximal in the class of prime zfilters on X contained
properly in Z[Mp]. ,.^
Theorem 4.2.5. Let p be a nonisolated Gfpoint of X. The point p is ramified if
and only if X \p is not C* embedded in X.
Proof: Let p be a nonisolated G^point of X. If p is not ramified then the prime
zideal P = Y!,QeUm(M ) Q ^^ properly contained in Mp. We will show, in this case,
that every point of X is the limit of a unique zultrafilter on X\p. Then by [GJ,
6.4] ,X\pis C*embedded in X.
Let q € X \ p. Then M, € Max(C(X \ p)) gives rise to the zultrafilter
Uq — Z[Mg] on X \p. Clearly q e n{clx{U) : U G Ug}. The uniqueness of W, is a
standard result [GJ, 3.18].
By [GJ, 6.3(b)], there exists a zultrafilter If on X\p converging to p. Assume
that there exists another such zultrafilter, V. Let U = jU and V = 7V. Then
Qu = Z^U and Qv = Zj[V are prime zideals of C(X) which are properly contained
in Mp. liQu^P then U = Zx[Qu] § Zx[P]. Hence U = j'^U g ^^Zx[P], which
contradicts that W is a zultrafilter on X\p. Likewise, Qv is not properly contained
in P. Thus P C Qu, P CQv and by [GJ, 14.8(a)], we must have either Qu C Qv
or Qv C Qu But Qu C Qv gives that W C V and therefore U QV. Since W is an
ultrafilter, W = V, as desired. In a similar manner, if Qy C Qu, then V = U.
Conversely, assume that p is ramified. Then Mp = Yj Min(Mp) and there
exists more than one prime zideal in C(X) which is maximal in the class of prime
81
zideals properly contained in Mp. These give distinct prime zfilters on X which
axe maximal in the class of prime zfilters on X properly contained in Z[Mp]. Hence,
via 7, we have distinct ultrafilters on X \p converging to p. Again by [GJ, 6.4],
X\pis not C*embedded in X. ■
Corollary 4.2.6. If X is a metric space, then X is ramified.
Corollary 4.2.7. If X is first countable and p €^ X is nonisolated, then X \p is
not C* embedded in X.
Corollary 4.2.8. Ifp € X is a Gspoint and X\p is not C* embedded in X, then
rk{p) > 2.
Let X and Y be completely regular spaces which are not Pspaces and let
W = X xY. We conjecture that every nonisolated point of W is ramified. We use
Theorem 4.2.5 to deduce two partial answers to this question.
Proposition 4.2.9. Let X and Y be completely regular spaces and letW = X xY.
Let X e X and y e Y be nonisolated Gspoints and let p = (x, y) € W. Then
W\{p} is not C* embedded in W.
Proof: Since X is completely regular and z is a Gjpoint of X, {x} is a zeroset
of X. Say, {x} = Zxif) for some / e C{X). Then we have for E^ = {x} x Y,
E. = {x}xY = n^{Zxif)) = Zw{fonx)
where ttx denotes the natural projection from W onto X. Likewise, we have that
Ey = X X {y} = Zw{g o TTy) where g e C{Y) such that Zy(g) = {y} and Try is the
natural projection from W onto Y. Let / and 5^ denote the restrictions of forcx and
gony to W\{p}. Then we have that E^\{p} = Zw\{p}{f) and ^„\M = Zw\ip}ig)
are disjoint zerosets of W\{p}. Thus Ex\{p} and Ey\{p} are completely separated
in W\{p}. But p € clwiE^\{p})ndw{Ey\{p}) and therefore ^x\M and Ey\{p}
82
axe not completely separated in W. By the Urysohn Extension Theorem, W \ {p}
is not C*embedded in W^. ■
Corollary 4.2.10. Let X and Y be completely regular spaces and letW = XxY.
Let X e X and y eV be nonisolated Gspoints and let p = (x, y) €. W. Then p is
ramified inW.
Proof: Follows from Theorem 4.2.5. ■
Proposition 4.2.11. Let X and Y be completely regular spaces and letW = XxY.
Letp= {x,y) €W be nonisolated. IfW\ {p} is normal, then W\{p} is not C
embedded in W.
Proof: Let Ex = {x}xY and Ey = Xx{y}. Then E^Xip} and Ey\{p} are disjoint
closed sets in the normal space W \ {p}. Thus, they are completely separated in
W \ {p}. But p e dw{Ex \ {p}) n clw{Ey \ {p}), so E^ \ {p} and Ey \ {p} are not
completely separated in W. Therefore, by the Urysohn Extension Theorem, W\{p}
is not C*embedded in W. ■
Corollary 4.2.12. Let X and Y be completely regular spaces and letW = X xY.
Let p= {x,y) eW be a nonisolated Gspoint of W. IfW\ {p} is normal, then p is
ramified in W.
Proof: Follows from Theorem 4.2.5. ■
We now investigate the ramification of G^points in a class of spaces more
general than metric or first countable spaces. ^.^
A topological space X is countably tight if for a subset U C X we have that
any p 6 d{U) is in the closure of a countable set S CU. A Frechet Urysohn space is
one in which every p e cl{U) is the limit of a sequence of distinct points {pn} C U.
It is evident that any Frechet Urysohn space is countably tight.
83
Lemma 4.2.13. Let X be a noTTnal topological space and letp€ X be nonisolated.
Then X\{p} is C* embedded in X if and only ifp^ dx{A) n dx{B), whenever A
and B are completely separated in X \ {p} .
Proof: By the Urysohn Extension Theorem, X \ {p} is C*embedded in X if and
only if A and B are completely separated in X, whenever A and B are completely
separated in X \ {p}. Assume that X \ {p] is C*embedded in X and let A and B
be completely separated in X \ {p}. Then A and B are completely separated in X
and hence are contained in disjoint closed sets of X. Thus p ^ dx{A) H dx{B).
Conversely, let A and B be completely separated in X\{p}. We wish to show
that A and B are completely separated in X. By hypothesis, p ^ dx{A) n dx{B).
Thus, dxiA) and dx{B) are disjoint closed sets of the normal space X. Hence, A
and B are completely separated in X. ■
Proposition 4.2.14. If X is a normal countably tight topological space andp £ X
is nonisolated, then X \ {p} is C* embedded in X if and only if for every two
countable sets 5i and S2 which are completely separated in X \ {p}, we have that
p^clx{Si)ndxiS2).
Proof: Assume that X \ {p} is C*embedded in X and let Si and 5^2 be two count
ably infinite sets which are completely separated in X\{p}. Then by Lemma 4.2.13,
we have that p ^ dx{Si) dx{S2)
Conversely, let A and B be completely separated in X \ {p}. Assume that
p € dx{A) n dx{B). Then there exist countable sets Si C A and S2 C B such that
p € dxiSi) n dx{S2) Since A and B are completely separated, so are Si and 52.
This contradicts the hypothesis. Thus by Lemma 4.2.13, A and B are completely
separated in X and X \ {p} is C*embedded in X. ■
84
Corollary 4.2.15. Let X be normal FrechetUrysohn space and letp £ X be non
isolated. Then X\{p} is C* embedded in X if and only if there do not exist two
sequences in X which are completely separated in X\ {p} and converge to p.
Finally, Proposition 4.2.14 and Theorem 4.2.5 imply the following.
Corollary 4.2.16. If X is a normal countably tight topological space andp G X is
a nonisolated Gspoint, then p is ramified in X if and only if for every two countable
sets Si and S2 which are completely separated in X \ {p}, p ^ clx{Si) O dx{S2)
4.3 Ramification via CEmbedded Subspaces
Let A and B be commutative rings with identity. Assume that we have a
surjective ring homomorphism f : A^ B, with K = Ker{f). Recall that there is a
onetoone orderpreserving correspondence between Spec(B) and the set of prime
ideals P e Spec{A) such that K C P. Let M e Max(B) be such that M = Fi + F2
for some nonmaximal proper primes Pi, P2 ^ Spec (5). If AT g Spec (A) corresponds
to M, then we have B/M ^ {A/K)/{N/K) ^ A/N. So N e Max(A). Let QuQ2
be the prime ideals of A corresponding to Pi and P2. Then (Qi + Q2)/K = Pi  F2
via the surjective map given by a\b i^ f{a)\f{b) with kernel K. Thus, by the
correspondence, we have that N/K ^ M = P1IP2 = {Qi+Q2)/K and TV = Qi^Q2.
In fact, we have shown: ]
Proposition 4.3.1. Let A and B be commutative rings with identity such that there
exists a surjective ring homomorphism f : A—^ B with K — Ker{f). If P < B is a
prime ideal which is a sum of two distinct prime ideals then there exists Q 6 Spec (A)
such that Q/K = P and Q is a sum of two distinct prime ideals of A.
Ramification in a Cembedded subspace implies global ramification.
Corollary 4.3.2. Let X be a completely regular space and let Y be a Cembedded
subspace. If a point p ofYis 2limbed in Y, thenp is 2limbed in X.
85
Proof: Since Y is Cembedded in X, we have a surjective ring homomorphism
from C{X) onto C{Y), given by restriction with kernel {/ G C{X) : Y C Zx{f)}
Hence this result follows from Proposition 4.3.1. ■
Proposition 4.3.1 also gives the following, since any compact subspace of a
completely regular space is Cembedded.
Corollary 4.3.3. 1. Let X be compact, Y C X a closed subspace. If a point of
Y is 2limbed in Y then it is 2limbed in X.
2. Let X be a compact space consisting of more than one point. If every noniso
lated point in a proper zeroset of X is 2limbed, then every nonisolated point
of X is 2limbed.
Example 4.3.4. The Cantor Set is metric, hence every point is 2limbed. By [E,
3.12.12c], every point of 2"^, the Cantor space of weight r, is contained in a closed
set which is homeomorphic to the Cantor Set. Thus every nonisolated point of 2^
is 2limbed. In fact, induction on Proposition 4.3.1 gives that every point of the
Cantor space is nlimbed for every n € N. D
Conversely, if a maximal ideal of A containing AT is a sum of primes containing
K, then by the correspondence given above, its image is a maximal ideal which is a
sum of primes in B. That is, if iV G Max(>l), K C N, ajid N = Qi\Q2 such that
KCQiajudKC Q^, then f{N) e Max(B) as B/f{N) ^ {A/K)/{N/K) ^ A/N.
And /(AT) = /(Qi + Q2) = /(Qi) + /(Q2) ^ Qi/K \ Q^/K.
This gives a partial converse:
Proposition 4.3.5. Let A and B be commutative rings with identity such that there
exists a surjective ring homomorphism f : A ¥ B with K = Ker{f). Let P be
a prime ideal of A containing K. Then P is a sum of two distinct prime ideals
86
containing K if and only if f{P)/K is a prime ideal which is a sum of two distinct
prime ideais of B.
Corollary 4.3.6. The ringhomomorphic image of a commutative ramified ring
with identity is ramified.
Corollary 4.3.7. Let Y be a Cembedded subspace of X and let p eY have finite
rank in X.Ifpis ramified in Y, then p is ramified in X. If any function in C{X)
vanishing on Y also vanishes on a neighborhood of p, then p is ramified in Y if and
only if p is ramified in X.
Proof: The first statement is an application of Proposition 4.3.1 by induction. The
second statement follows from Proposition 4.3.5 by induction. ■
Note that the hypothesis of the second statement of the above merely de
mands that the kernel of the restriction map be contained in O^ . This is satisfied
if Y is open or if F is a Pset. The preceding results indicate that ramification is a
local property.
Let A be a commutative ring with identity, let 5 be a multiplicative system in
A such that 1 G 5. Then there exists a onetoone correspondence from Spec(5'~^>l)
onto {P € Spec(>l) : POS = 0}. The proofs of the following are routine:
Proposition 4.3.8. Let A be a commutative ring with identity, let S be a multi
plicative system in A such that 1 G 5.
1. IfP€ Spec(5'~M) is a sum of two proper primes, then the preimage ofP, the
set {x € A : x/1 E A}, is a prime ideal which is a sum of two proper primes
, in A.
2. If Q E Spec (A) is a sum of primes and Qd S = 0, then S~^Q is a sum of
primes in S~^A.
87
Corollary 4.3.9. Let Y be a subspace of X such that C{X) ^ C{Y) is a ring of
fractions map. That is, there is a multiplicative system S C C{X) such that 1 G 5
and C{Y) = S'{C{X)). Then:
1. Forp G Y, rkvip) < rkxip)
2. Let peYIfM}nS = 0, then there is P e Spec(C(X)) such that P^M}
and rkvip) =rkx{P)
» ;
CHAPTER 5
mQUASINORMAL /RINGS
In [Lai 3], Suzanne Larson defines the notion of a quasinormal f ring; one in which
the sum of any two distinct minimal prime ideals is a maximal ^ideal or the entire
/ring. We generalize this definition and a few of her results.
5.1 Definitions
Definition 5.1.1. Let A be a commutative fring ivith identity and let m be a
positive integer. Call A mquasinormal if the sum of any m distinct minimal prime
ideals is a maximal iideal or the entire fring A. If X is a topological space such
that C{X) is mquasinormal, then we call X an F^space.
Note that the "2quasinormaJ" is Larson's "quasinormal" condition, the "1
quasinormal" condition is equivalent to von Neumann regularity, and if A is m
quasinormal then A is nquasinormal for any n > m. Hence, the Fjspaces are
exactly the Fspaces and any F^space is an F„space, when n> m.
Theorem 5.1.5 generalizes [Lai, 3.3] and characterizes the rriquasinormal
semiprime /rings. Note that [Lai, 2.2], which we now state, gives necessary and
sufficient conditions for a semiprime /ring to have the property that the sum of
any two distinct minimal prime ideals is a prime ^ideal. This condition is stronger
than the assumption we make in our theorem, but this result indicates when one
can expect to be able to apply it.
Theorem 5.1.2. Let A be a semiprime fring. The following are equivalent:
1. The sum of any two semiprime £ideals is semiprime.
88
89
2. The sum of any two minimal prime £id&ils is prime.
3. The sum of any two prime £ideals is prime.
4 For any a,b e A^, the £ideal a'' + ft*^ is semiprime.
5. For any two disjoint elements a,fe e A+, the iideal a^ •{■h^ is semiprime.
6. For any a, 6 e A"*", the iideal a^ + b^ is semiprime.
7. When x, a, &, c, rf 6 A+, a, & 7^ are such that x^ = c + d and aAc = bAd = 0,
there exist g,h€i A such that x = g + h and gAa = hAb = Q.
The theorem above holds for C(X) since the sum of two prime ideals is prime
by [GJ, 14B]. We will use the following lemmas. The first follows from the fact that
a prime ^ideal P of a semiprime /ring is minimal if and only if for every p ^ P
there exists q ^ P such that pq = 0. The second lemma shows the existence of
certain functions which we will take for grafted in the proofe to follow.
Lemma 5.1.3. Let A be a semiprime fring in which the sum of any m distinct
minimal prime ideals is a prime £ideal. Let P be a prime iideal. Then P is
minimal with respect to containing ]C^i ^f */ °"^ ^^^J/ */ f^^ every p £ P there
exists q ^ P such that pq G YlJ=i ^f
Lemma 5.1.4. Let A be a commutative fring. Let n > 2 and let Qi,...,Qn be
distinct minimal prime ideals. Then there exists an element f 6 Qj*" \ [Jj^Qj, for
eachi = l,...,n.
Proof: Let fk € Qt \Qk for A; = l,...,n. Then, by convexity, we have that
/ = VL2/ieQMUi^i<5i, as desired. ■
Theorem 5.1.5. Let m be a positive integer. Let A be a commutative semiprime
fring with identity in which the sum of any m distinct minimal prime ideals is a
prime iideal. The following are equivalent:
90
1. A is mquasinormal.
2. For every nonmaximal prime £ideal P, rk(F) < m — 1.
3. Let {aj}^i be a family of positive pairtvise disjoint elements of A. Proper
prime iideals containing Y^=i ^f ^''^ maximal iideals.
4 Let {aj}p.i be a family of positive pairwise disjoint elements of A, let M be
a maximal iideal containing the iideal ^17=1 °'fi '^^^ ^^* p € M. Then there
exists z ^ M such that zp G Y^JLi ^f
Proof: (1) =>• (2) : Let P be a nonmaximal prime ^ideal of A such that rk(P) > m
and let Qi, . . . , Qm be m distinct minimal prime ideals that are contained in P. Then
YlTi Qj ^ ^ is not maximal, hence A is not mquasinormal.
(2) =» (3) : Let P be a prime ^ideal containing Ylfi af. Then afCP for every
j. Therefore, Cj ^ 0{P) for all j, and hence P contains at least m minimal prime
ideals by the pigeonhole principle. Thus condition (2) gives us that P is a maximal
^ideal.
(3) <=^ (4) : Follows from Lemma 5.1.3.
(3) ^ (1) : Let M be a maximal ^ideal and let Qi, • • ■ , Qm C M be minimal prime
ideals. Then by hypothesis, Y^JjLiQj is a prime ^ideal and we are left to show
that it is a maximal ^ideal. For each j = 1,.. .m, let Gj E Q'^\ lii^jQi and define
hj = Aj^ijOj — AJt^iCife G Y^=i Qj Then {6j}^i is a pairwise disjoint set of m distinct
elements of Ylf^i Qj and by the choice of the Oj's, we have that AjfeLiO/t ^ ^T=iQk
by convexity and Aj^jOj ^ Qj for each j. Hence bj ^ Qj and bf C Qj, for each j.
Thus X)^i Qj is a maximal ^ideal by condition (3), since E^i bf C ^^^ Qj. m
The quasinormal condition is a variation of the normal condition, which is
that the sum of any two minimal prime ideals of a semiprime /ring with identity
is the entire /ring. This is discussed in [Hu]. The expected generalized definition
91
follows, along with a theorem recording two equivalent conditions. The result is a
special case of Theorem 5.1.5 and the proof follows immediately from the one above
and from [Hu, Theorem 8].
Definition 5.1.6. Letm>2 be a positive integer. An fring A is mnormal if for
any pairwise disjoint family {aj}!^i, we have that A = Ylj=i ^j ■
Theorem 5.1.7. Let A be a commutative semiprime fring with identity and let
m>2 be a positive integer. The following are equivalent:
1. A is mnormal.
* 2. For any maximal tideal M, we have that rk(M) < m — 1.
S. The sum of any m distinct minimal prime ideals is A.
Before we move to describe F^spaces, we first discuss a special class of
Tnquasinormal /rings.
Definition 5.1.8. Let A be a local fring. An embedded prime £ideal P is high
if for every minimal prime ideal N e Min(A), either NCPorN\/P = lex{A).
Otherwise, P is low. Call an fring A a broom ring if for every maximal iideal M
every prime iideal in Am is high.
The following is immediate from Proposition 2.5.2 and Lemma 5.1.3.
Proposition 5.1.9. Let A be a local commutative semiprime fring with identity
and maximal iideal M. The following are equivalent:
.• If r' .1
1. A is a broom ring.
2. T^{A) < 2.
3. If P C lex{A) is a prime iideal, then rk^(P) < 1.
92
4 If a,b G A are disjoint and P is a proper prime (.ideal containing a^ + b^,
then lex(A) C P C M.
5. Ifa,beA are disjoint and a^ + b^ C lex(A), then for every p G \ex.{A), there
exists z ^ lex(A) such that zpea^ + b^.
Proof: (1) "^ (2) : Since every prime ^ideal of A is high, we have that for any
two distinct minimal prime ideals QuQ2 that Qi V Q2 = lex(A). Hence 7r(A) < 2
by definition. Conversely, Tr{A) < 2 implies that every minimal prime ideal is high
and therefore that every prime ^ideal is high, as desired.
(2) •» (3) : Since 7r(yl) < 2, we have that rk(P) < 1 for any embedded prime ^ideal
P, by Proposition 2.5.2; and vice versa.
(2) => (4) : By Proposition 2.5.2, we have that a^ + b^ = \ex{A).
(4) <^ (5) : Follows from Lemma 5.1.3.
(4) =^ (3) : Assume that (4) holds. Let P be an embedded prime ^ideal and assume,
by way of contradiction, that rk(F) > 2. Let QuQiQ P he minimal prime ideals.
Let qi € Qt \ Q2 and let q2 ^ QtXQi Disjointify by defining q] = Qj  Qi /\ 92
for j = 1,2. Then q^^ C Q2 since q{ ^ Q2 and ^^ C Qi since qi ^ Qi Then
q{^ + ^^ C P and hence lex (A) C P, which contradicts that P is embedded. ■
Ex£iinple 5.1.10. We now present an example of an /ring which is 3quasinormal
but is not a broom ring. Let Aq = Ai = [0, 00) C R, and A2 = A3 = (1, 00) C R,
where each is inversely ordered. Identifying the copies of in the disjoint union
A^ = (Aq U Ai)/(Oo ~ Oi) and letting A^ be the disjoint union A2 U A3, we obtain
a root system A = A^ U A^ with the induced ordering which we now describe. That
is, r < s if and only if either r, s G Aj for j = 0, 1, 2 or 3 and r < s in the inversely
ordered real numbers; or if r 6 A2, s 6 Ai; or if r 6 A3, s G Ai. Explicitly, r  s if
r 6 A2 and s € Aq n A3 or if r 6 Ai and s e Aq.
93
We endow A with a partial binary operation. To begin, note that we define
Oo + Oo = Oi + Oi = Oo + Oi = Oo ' Oi. Let Tj, Sj £ A^ be nonzero for j = 0, 1, 2, 3.
Let (*)j denote the sum in parentheses as the usual sum of real numbers residing in
Aj] the mark "" signifies that the sum is undefined.
+
ro
so
ri
SI
r2
S2
r3
S3
ro
(2ro)o
(ro + so)o




—
—
so
(ro + so)o
(2so)o
—



—
—
ri
—
(2ri)i
(si +ri)i
(r2 +ri)2
(S2+ri)2
(r3 + ri)3
(s3 + ri)3
SI
_

(ri+si)i
(2s, )i
(r2+Sl)2
(S2 + Sl)2
(r3 + Si)3
(S3 + Si)3
r2
_
—
(ri + r2)2
(si +r2)2
(2r2)2
(S2+r2)2

—
S2
_
—
(ri + S2)2
(si +82)2
(r2 + S2)2
(2S2)2


rs
_
—
(ri + r3)3
(si +r3)3


(2r3)3
(S3 + r3)3
S3


(ri + 83)3
(Sl + S3)3

—
(r3 + S3)3
(2S3)3
Let Co = Ao,Ci = Ai,C2 = Ai U A2, and C3 = Ai LI A3. Then the minimal prime
ideals of F(A,R) are of the form Qj = {u e F(A,R) : supp(m) C A\Cj} for
j = 0,2,3. The similarly defined Qi is a prime ideal by Proposition 3.3.15. Now,
it is evident that L = lex(F(A, R)) = Qo V Q2 V Q3 and Q2 V Q3 = Qi / i so we
know that 7r(F(A,R)) = 3. Therefore, Proposition 5.1.9 shows that the ring is not
a broom ring. Since L is the maximal ideal of the ring, we have shown that F(A, R)
is 3quasinormal. D
We present a similar example of an broom ring that is not quasinormal.
Example 5.1.11. Let Ao = [0,oo) C R, and Ai = A2 = (l,oo) C R, where each
is inversely ordered. Let A^ = Aq and let A^ be the disjoint union Ai U A2 in
order to obtain a root system A = A^ U A^ with the induced ordering which we now
describe. That is, r < s if and only if either r, s e Aj for j = 0, 1 or 2 and r < s
in the inversely ordered real numbers; or if r e Ai,s G Aq; or if r € A2,s € Aq.
Explicitly, r  s if r € Ai and s e A2.
We endow A with a partial binary operation. Let rj, Sj € Aj for j = 0, 1, 2.
Let (*)j denote the sum in parentheses as the usual sum of real numbers residing in
Aj; the mark "— " signifies that the sum is undefined.
94
'0
ri_
«i
«2
ro
(2ro)o
(rp + ao)o
(ro + ri)i
(rpf8i)i
(rp + rzja"
{ro + S2)2
so
l»P
+ rp)p
(2ao)o
(ao + ri)i
(ap + ai)i
(so + Ti)^'
[SQ + S2)2
n
(ri +ro)i
(ri + ao)i
(ri + 8i)i
»l
(ai + rp)!
(ai +ao)i
(ai +ri)i
1270T
r2
(r2 +rp)2
(rz +ap)2
12^2)2
(r2 + 82)2
a2
(a2 + ro)2
(aa +ao)2
(a2 +r2)2
(2S2)2
Let Co = Ao,Ci = Ao U Ai and C2 = Ao U A2. Then the minimaJ prime ideals of
F(A,R) are of the form Qj = {u e F(A,R) : supp(«) C A \ Cj} for j = 1,2; the
ideal Qo, is a prime ideal by Proposition 3.3.15. Now, Qo = lex{F(A, R)) = Qi V Q2
and so we know that 7r(F{A,R)) = 2. Therefore, Proposition 5.1.9 shows that the
ring is a broom ring. Since Qo is not the maximal ideal of the ring, we have shown
that F(A, R) is not quasinormaJ. D
5.2 (B, m)Boundarv Conditions
Definition 5.2.1. Letm be an integer greater than 1 and let {Uj}]^^ be a family of
m pairwise disjoint cozerosets of a topological space X. The subspace C(^=idxiUj)
is called an mboundary in X. Let B be a topological property. We say that a space
X satisfies the (B, m)boundary condition if every mboundary in X has property
3. , ^ . , 
In [Lai, 3.5], Larson proves that ii X is completely regular, every point of X
is a Grpoint, and C{X) is quasinormaJ, then X satisfies the (discrete, 2)boundary
condition. This result is improved in [La3, 3.5] to say that if X is normal and for
every p 6 0X \ vX, the ^ideal O"' is prime then C{X) is quasinormal if and only if
X has the (finite, 2)boundary condition. Here we refine this theorem by removing
the restriction on the points of the corona.
First, a lemma, extending [La3, 3.1], which we henceforth refer to as "Larson's
Lemma" :
Lemma 5.2.2. Let X be normal and let {j9j}!^i Q C(X)'^ be a family of pairwise
disjoint functions. Define Y^ = 07=1 dx{coz{gj)). Then Y^^Li of = Ciy^y^ My.
95
Proof: Each function in Y!^^i of must vanish on F^, hence YjJ^x df ^ (^y^Vm ^v
For the reverse inclusion, we use recursion. The proof of the base case of m = 2 is
in [La3, 3.1].
Let / G riyey^ ^y ^^ define
/•/\^//^l ifxecZ(co2;(5i)),
Since X is normal and f\ is defined on a closed set, the function has a continuous
extension, /i € C{X). Then / A 1 /i 6 5i^ and 7i e fK^^p = P e 0^2 c/(co2:(5j))}.
Recursively define a function /n. to be the continuous extension of
^*^"^  \0 if
X £ d{coz{gk))
xer\]lk^,cl{coz{gj)).
Then 7fc_i A 1  7fc G ^j^ and 7fe G fli^P = P ^ fl^it+i c^(«>^(5i))} In particulai, by
the base case, we have that 7m2 € C\{^p '■ P ^ PijLmi ^i<^^i9j))} = 9mi + 9m
But then 7m3 ^^ 1  7m2 e ffm2 implies that 7m3 ^ 1 G YlT=m2 9f and therefore
we have that 7m3 = (7m3 A l)(7m3 V 1) G YJj=m29f Thus, by recursion, we
deduce that 7i G E^2^/ Hence /Al = (/Al7i)+7i ^ E^i5i^ and therefore,
/=(/ A l)(/Vl)GEr=i^^ as desired. ■
Let X be normal and let K^, as above, be given. The set Y^ is Cembedded
in X by [GJ, 3D]. Thus we have a surjective ring homomorphism ip : C{X) » C(y^)
given by restriction of functions. The kernel of the homomorphism is
171
K = {feC{X):Yr.CZx{f)}= n Mv = Y.sf^
by Larson's Lemma. Thus by the First Isomorphism Theorem, it follows that
m
C{Y^)<^C{X)/K = C{X)lY,9f
We utilize the onetoone correspondence between the prime ideals of CiXrr^ and
prime ideals of C{X) which contain K, the kernel of v
96
Theorem 5.2.3. Let X be normal and m > 2 an integer. The ring C{X) is m
quasinormal if and only if X satisfies the {P,m) boundary condition.
Proof: Let Y^ = fl^i cl{coz{9j)) be an mboundary. Y^ is a Pspace if and
only if the prime ideals of C{Ym) are both minimal and maximal by [GJ, 4J]. By
the discussion above and Larson's Lemma, this is also equivalent to the condition
that the prime ideals of C{X) containing YJ^liOf ^^ maximal. In turn, this is
equivalent, by Theorem 5.1.5, to the statement that C{X) is mquasinormal. ■
By [GJ, 4K.1], we have that countably compact Fspaces must be finite and
discrete. In this light, the following is immediate.
Corollary 5.2.4. Let X be normal.
t.; :• ...
1. If X is countably compact, then C{X) is mquasinormcU if and only if X
satisfies the {&nite,Tn) boundary condition.
2. If X is locally compact, then C{X) is mquasinormal if and only if X satisfies
the {discrete, m) boundary condition.
3. If X is acompact, then C{X) is mquasinormal if and only if X satisfies the
(countable discrete, 7n)6o«nrfan/ condition.
Example 5.2.5. (Butterfly Spaces) Let X be a noncompact completely regular
space and let S C px \X he closed. Define
m
Brr.{X,S) = (]j0Xj)/iS,r^S2 Sm),
J=l
where we assume that Xj = X,Sj = S for all j = 1, . . . m and Si ~ Sj indicates
that corresponding points of S are identified, when i ^ j.
Consider X = B„(N, /?N \ N). Let gj{x) = ^ when x eNj and let gj vanish
elsewhere on X. Then {gj}j^i is a pairwise disjoint set of functions such that we
97
have y„ = nr=i dx{coz{gj)) = /?N \ N, which not a Pspace. Thus B^(N, /?N \ N)
is not an F^space by Theorem 5.2.3. Every point of this space has rank less than
or equal to m, thus it is an F^+ispace, by Theorem 5.1.5.
The minimal prime ideals contained in Mp for p € /9N \ N and j = 1,... ,m,
are given by:
Qj = {/ e C(B^(N,/3N \ N)) : p 6 m«^N,ci^N,^N, (/In,)}.
We show that C(B,„(N, /?N\N)) is a broom ring by demonstrating that Qj, C Qj+Qj
for all j > 3. Let f ^ Q^ and represent the function by an mtuple (/i, /z, • • . , fm)
where fk = /I/jn* for all A; = 1, . . . , m. For some j > 3, let
9i = {fjj2,f3,,fm) and 92 = {fi fj, 0,0,..., 0).
Then ft e Qj, for i = 1, 2 and / = 51 + 52 G Qp + Qp Thus we have shown that
7r(C(B^(N, (m \ N))mp) < 2 for all p G B^(N, /?N \ N). Hence C(B„(N, /?N \ N)) is
a broom ring by Proposition 5.1.9.
Let (f : C{Bm{^, /3f^ \ N)) > C{Ym) be the canonical surjection with kernel
K = YlTidf' ^® want to explicitly describe the root system structure at each
point of the space 5m(N, /?N \ N). For each j, the points p 6 Nj are isolated and
hence the spectrum at each of these points consists only of the maximal ideal. If
p e ^N\N is a Ppoint then Mp E Max{C{Ym)) is minimal. Hence Mp = (p^{Mp) is
a maximal ideal of C(J3m(N, /3N \ N)) and is minimal with respect to containing K.
Thus, the maximal ideal Mp is ramified and the graph of Spec(C(PTO(N, /?N\ N)jg )
has the form:
(5.1)
98
Ifpe/9N\Nisan Fpoint which is not a Ppoint, then Mp € Max(C(Fm))
properly contains a unique minimal prime ideal, namely Op. Hence Mp = (p^{Mp)
is a maximal ideal of the ring C{Bm{N, pN \ N)) and the ideal Op = (f'^iOp) is the
unique prime ideal of C(B^(N, /?N\N)) which is minimal with respect to containing
K. Thus, Ql+Ql = DJLi Q^ = Op S Mp and the graph of Spec(C'(5„(N, /5N\N)j^J
has the form:
(5.2)
The following is also immediate from Theorem 5.2.3:
Corollary 5.2.6. Let X he normal.
1. Let X he a noncompact, locally compact, acompact Fspace and S a closed
suhspace of the corona. Then B2{X, S) is an Fspace if and only if S is finite.
2. Let X he an Fspace and S a nowhere dense zeroset of X. Let Xj = X and
let S = Sj C Xj for j = 1,2. Define Y = {X^U X2)/{Si ~ S2), where
corresponding points of S are identified. Then Y is an F^space if and only if
S is a Pspace in the suhspace topology.
Proof: (1) By [GJ, 6.9], the hypotheses on X imply that fiX \X is compact and
A" is a cozeroset of PX. Thus S is compact and there exists / G C{I3X) such that
coz{f) = X. Let
' f{x) if X e pXi ^ ,_, j f{x) if xe 13X2
/i(x) =
t
if X 6 X2
^W = {r
iix e Xi
99
Then d{coz{fi)) n d{coz{f2)) = S. If B2{X, S) is an Faspace, then 5 is a compact
Pspace by Theorem 5.2.3. Thus, S is finite by [GJ, 4K.1].
Conversely, assume that S is finite. Since X is an Fspace, we know that
any 2boundary of B2{X,S) must be contained in S and must therefore be finite.
Thus, every 2boundary is a Pspace and we conclude that B2{X, S) is an Paspace
by Theorem 5.2.3.
(2) As above, any 2boundary of Y is contained in S. In particular, 5^ itself is a
2boundary since it is a nowhere dense zeroset of X. Thus, if y is an Paspace, then
5 is a Pspace, by Theorem 5.2.3. Conversely, since any subspace of a Pspace is a
Pspace by [GJ, 4K.4], we know that every 2boundary of F is a Pspace. Therefore,
Y is an Paspace. ■
Next, we show that it is the case that any normal Pspace arises as a 2
boundary in an Paspace. " "' ' ; " ;; j. •? i • ■
 ^. ... ■.. ■ ....
Proposition 5.2.7. LeX Y he a normal Pspace. Then there exists an F2space X
containing Y such that in the subspace topology Y = clx{coz{fe)) clx{coz{fo)),
where fe,fo £ C{X)'^ are disjoint.
Proof: Let F be a normal Pspace and for each y G F, let ayNy be the onepoint
compactification of the natural numbers in which ay plays the role of the point
at infinity. Define X = (Y U iUy^Y ^v^v)) / i^v ~ v) ^^^ *^^ points of N remain
isolated. A base for the ^neighborhoods of y G F is given by the sets of the form
^y U (Uj/'eu ^y*)) where Uy is a neighborhood of y in F and Ny> is a neighborhood
of ttj,' in OCy''Ny'.
Define fe on X such that /e(n) = ^ for all even n £ Ny and fe{x) =
otherwise. Similarly, define a function /„ on X such that foin) =  if n G Ny is odd,
and foix) = otherwise. Then fe^fo = and F = dx{coz{fe)) n dx{coz{fo)).
100
Since the only points of X having rank greater than 1 are contained in the
subspace F, we have that all the 2boundaries are subspaces of a Pspace. Thus by
[GJ, 4K.4] and Theorem 5.2.3, we have that X is an F2space. ■
It is natural to consider the implications of boundary conditions other than
for Fspaces. We now take a look at Fspace boundary conditions.
Theorem 5.2.8. Let X be normal. The following are equivalent:
1. X satisfies the (F— space, n)6o«ndarj/ condition.
2. Let p e fix. If {Pi, ... , P2n} ^ MP are pairwise incomparable prime ideals,
then X)j=i Pj '^"'^ J2j=n+i Pj °^^ comparable.
3. There does not exist a pair of noncomparable prime ideals contained in the
same maximal ideal such that each has rank greater than or equal to n. That
is, the graph of Spec(C(X)) does not contain a copy of
Proof: The equivalence of (2) and (3) is clear.
(1) ^ (3) : Let p e 0X be such that M^ contains two incomparable prime ideals,
F, Q, each of rank greater than or equal to n. Let F^, . . . , F„ C F and Qi, ■■■,QnQQ
be distinct minimal prime ideals. For A; = 1, . . . , n, let
/ifc e Qt \ {{Uj^kQj) u (Ui^kPi)) f2k e Pi: \ ii^j^kQj) u {\Ji^kPi)).
Then by convexity and primeness, fk = /ifcA/afc G {Qtf^Pk)\iiUjjtkQ3)^{l}ijtkPi))
We disjointify by defining gk = Aj^fe/i  /^%ifj Then gk ^ Qk^ Pk and hence,
9i c Q, n Pk. Thus ELi ^i^ ^ (ELi Qk) n (ELi Pk)QPn Q.
101
If y = Q"^^ d{coz{gk)), then by the prime correspondence discussed before
Theorem 5.2.3, and by the construction of the functions {gkjk^^i, the incompa
rable prime ideals P and Q of C{X) give rise to the incomparable prime ideals
PlYrk=i9i,QIY.l=i9i Q MVELi5i^ i« C{Y). Therefore, y is an nboundary
which is not an Fspace.
(3) =^ (1) : Let F = n"=i c/(co2;(/j) be an nboundary in X and assume that Y
is not an Fspace. Then there exists p €. PV such that M^ contains incomparable
prime ideals P, Q. Since C{Y) ^ C{X)/ Yl%i fj, we have that M^ contains the two
incomparable prime ideals P, Q such that P/ Yl"=i fj = P and Q/ YTj=i //  Q
Thus, since YTj=i ft  ^' ^^ ^°°^ *^^* '"^c(x)(^) > ", by the pigeonhole principle
and [BKW, 3.4.12]. Likewise, rkc{x){Q) >n. m
Example 5.2.9. Consider the ordinal X = iJ^ Vim the interval topology. Let E
be the subset {A2n:nGci;,A limit ordinal} be the set of even ordinals less than
a;^ I 1 and let D be the subset {A I (2n t 1) : n € a;, A limit ordinal} be the odd
ordinals less than w^ ( 1. Define /e(x) =  if x = A I 2n £ F, and let /g vanish
otherwise on X. Let /o(x) = Mf x = Al (2nfl) € D and let /„ vanish otherwise on
X. Then Y = dx{coz{fe)) fl c/x(co2(/o)) = {no; : n e w} U {o;^} is homeomorphic
to the one point compactification of u where u^ acts as the point at infinity. This
2boundary is not an Fspace. Thus, via analysis similar to the proofs above, the
canonical surjection and [GJ, 14G], we know that the spectrum at A4,s in C{u^ + 1)
contains the following subgraph:
(5.3)
102
In fact, if, for m > 2, we partition N into m countably infinite sets {Aj}^^i and let
. , . \  if X = A + n for n e A j
^W=o else.
Then Y = fl^i cl{coz{fj)) = {nu : n G a;}U{a;^} is an mboundary in w^ + l. Thus
each branch in the graph above that emanates from the maximal ideal has at least
2' minimal vertices, by Proposition 2.3.3. D
The special Fspaces, extremally and basically disconnected spaces, have a
different sort of boundary description. Let us review a couple of results. Recall that
a completely regular space X is cozero complemented if for any cozeroset U C X
there exists a cozeroset V C X such that U DV = and U UV is dense in X.
Dually, an /ring A is complemented if for every f G A^ there exists g E A'^ such
that f Ag = Q and f + g is not a zerodivisor.
Lemma 5.2.10. Let X be completely regular.
1. The following are equivalent:
(a) C{X) is complemented,
(h) X is cozerocomplemented.
(c) Min(C(X)) is compact.
2. X is a cozerocomplemented F space if and only if X is basically disconnected.
3. X is extremally disconnected if and only if Min{C{X)) = Max(C(X)) is ex
tremally disconnected.
Proof: (1) The equivalence of (a) and (c) is shown in [M, 1.5]. The equivalence of
(a) and (6) is from the definitions.
(2) See [HVWl, 2.16].
(3) See [M, 2.6] and [M, 2.7]. ■
103
Let F C C{X). Under the hullkernel topology on Spec(C(X)), we define a
subspace homeomorphic to Spec(C(X)/ Ylf^p /"^) by
Vjr = {P e Spec(C(A")) : P is minimal with respect to containing ^ /"''}•
Proposition 5.2.11. Let X he normal. The following are equivalent:
1. X soiis^fts i/ie (basically disconnected, n)6oundan/ condition.
2. X satisfies the (cozero complemented F, n) boundary condition.
3. If F = {fj}]=i C C(X)+ is o pairwise disjoint family, then Vf is compact
and there does not exist a pair of noncomparable prime ideals contained in the
same maximal ideal such that each has rar^ greater than or equal to n.
Proof: (1) ^ (2) : Follows from (2) of Lemma 5.2.10.
(2) ^ (3) : The compactness of Vf follows by (1) of Lemma 5.2.10 and the prime
correspondence discussed before Theorem 5.2.3. The final statement of (3) is a result
of Theorem 5.2.8. ■
Proposition 5.2.12. Let X be normal. The following are equivalent:
1. X satisfies the (extremally disconnected, n) boundary condition.
2. If F = {/j}"=i C C{X)'^ is a pairwise disjoint family, then Vf is extremally
disconnected and compact and there does not exist a pair of noncomparable
prime ideals contained in the same maximal ideal such that each has rank
greater than or equal to n.
Proof: Follows directly from (3) of Lemma 5.2.10, the prime correspondence dis
cussed before Theorem 5.2.3 and from Theorem 5.2.8. ■
Next we describe the spectra of C{X) which guarantee the (F^, n)boundary
condition on X.
. \
104
Definition 5.2.13. Ze* a € N and letn>0 be an integer. Then we call the ntuple
a = (ai,02,...,an) a good (ordered) paxtition of a if the following hold: for all
j = l,...,n, we have that < %■ G Z, o = ai + 02 H f On, ai > 1 and ci > 02.
Construct an upwaxddirected graph Aq corresponding to a good partition a
of a containing: nodes Pi,P2,,Pn such that Pi < P2 < • • • < Pn, each pj lies above
5j = tti + 02 H \aj terminal vertices and there is a node q such that q > pj for
every j = 1, 2, . . . , n. As follows:
(5.4)
Proposition 5.2.14. Let A be a commutative semiprime fring with identity in
which the sum of any m distinct minimal prime ideals is a prime £ideal. Then A is
mquasinormcd if and only if Spec^(>l) does not contain A^ as a subgraph for any
good ordered partition aofm.
Proof: Assume that Spec^(>l) contains A^ as a subgraph for some good ordered
partition a = (oi, . . . , o„) of m. Let Pk be the prime ^ideal at the node pk for each
A; = 1, . . . , n. For a fixed k, let Qk = {Qkj}%i be a family of distinct minimal prime
ideals contained in Pk but not contained in Pj for j < k. Then [J"^^ Qk is a family
of m distinct minimal prime ideals whose sum is the nonmaximal prime ^ideal P„.
Thus A is not 7nquasinormal.
Conversely, if A is not mquasinormal, then there exists a family of m distinct
minimal prime ideals {Qk}^=i such that Y^^=i Qk is not a maximal ^ideal. Let
105
Pi = Yjk=i Qk ^^ ^^* ^ ^^ *^^ maximal ^ideal containing it. Then the subgraph
of Spec (A) bounded by the vertices corresponding to the prime ^ideals in the set
{Pi,M} U {Qfc}JbLi is of the form A^ for some a. m
Example 5.2.15. The good ordered partitions of 4 are (4), (3, 1), (2, 2) and (2, 1, 1).
UX is normal, then it satisfies the {P, 4)boundary condition if any only if Spec(C(A"))
does not contain any subgraphs of the following forms:
(5.5)
This follows from Theorem 5.1.5 and Proposition 5.2.14.n
Obtain the graph A* by appending a graph having at least A; > 2 minimal
vertices to each terminal vertex of the graph A^. For instance:
(5.6)
The following is a consequence of Proposition 5.2.14 and the method of proof
of Theorem 5.2.3.
Corollary 5.2.16. Let X be normal. Then the following are equivalent:
1. X satisfies the (Fm, k) boundary condition.
106
2. Spec(C7(A')) does not contain a subgraph of the form A* for any good ordered
partition a of m.
3. Let a family V C Spec(C(X)) ofmk noncomparable prime ideals and a good
partition a = (oi, 02, . . . , a„) ofmk be given. Partition V into pairwise disjoint
sets {Vj}]^i where Vj contains aj elements ofV. Then at least two of the prime
ideals given by ^ Vj are comparable. .
Proof: The equivalence of (2) and (3) is clear.
(1) => (2) : Assume that X contains a fcboundary Y = ff^j d{coz{fi)) which is not
an Fmspace. Then by Proposition 5.2.14, Spec(C7(y)) contains a subgraph of the
form Aa below some maximal ideal My for some good partition a = (ai, . . . , a„) of
m and some p E f3Y. For each i = 1, . . . , n, let Pj be the prime ideal at the node
Pi G Aq and let {Qj^}^i be a family of distinct minimal prime ideals contained in
Pi but not contained in Fj for / < i. Then, by the prime correspondence discussed
before Theorem 5.2.3, there exist, for each i, prime ideals Pi,Qij in C{X), such
that Pi ^ Pi/Eti fi^ and Qi^ ^ Qij/Eti ft for each 3. Then by [BKW, 3.4.12],
rk(Qi ) > k for all i, j. Therefore, the spectrum below M^ contains a copy of A*.
(2) =^ (1) : Assume that the spectrum below M^ contains a copy of A* for some
good partition a = (oi, . . . , a„) of m. For each i = 1, . . . , n, let Fj be the prime ideal
at the node Pi G A* and let {Qi }^i be a family of distinct nonmaximaJ as well as
nonminimal incomparable prime ideals contained in Fj, but not contained in Fj, for
/ < i. For each pair i, j such that \ <i <n and 1 < j < Cj, let {Q'.}f=i be a family
of distinct minimal prime ideals contained in Qi but not contained in Qr, , if r < i
or if r = i and s ^ j.
Let fstr £ Q\t \ (U i¥^ Q\t) Then, by convexity, we have that for all
l<a<n
l<t<a,
^ < r < k, fr = Mi<s<nfstr e {r[i<s<nQ\t) \ (U H^r Q[X Disjointify thesc
l<t<o, l<t<o, l<»<n
l<t<a,
functions via gr = A„^r/«  ^t=ifv t Q^at Then g^ C Q^^ for all s,l Finally,
107
Eri 9r ^ Qij for all pairs i,j. Then Y = ff^^ d{coz{gr)) is not an Fmspace since
M^ contains A^ as a subgraph, by our choice of the functions. ■
5.3 0X, mQuasinormal and SV Conditions
First we give equivalent conditions under which one may expect that /3X is
an Fmspace for some integer m. This improves [La2,4.3] and extends [La2, 3.3].
Lemma 5.3.1. Let X be normal and let Y^^ {x e X : rkx(x) > m  1}.
1. Let y^(3X\X. //rk^x(y) > m  1, then y 6 dpx{Yra)
2. IfYm is compact, then rk^xiy) <ml for every y € PX \ X.
Proof: (1) Let ?7 be a /?Xneighborhood of y. Then there exists a closed PX
neighborhood V of y such that V C.U and F n X is closed in X. Since V r\X is
C*embedded in X, we have that y e /?(y n X) = dpxiV r\X)CV. Now, by [La2,
1.6], we have that V n F^ y^ since rk^x(y) > m  1.
(2) Let y^(3X\X.Vi. rk^x(y) > m  1, then by (1), y e d^xiX^) Q X, which is a
contradiction. Hence, rk^x(F) < m — 1. ■
Using the lemma above and Theorem 5.2.3, we obtain:
Theorem 5.3.2. Let X he normal and let m>2 be an integer. The following are
equivalent:
1. C{X) is mquasinormal and Tkx{M^) <m—l for every p E jSX \ X.
2. X satisfies the {'&nite,Tn) boundary condition.
3. X contains only finitely many points of rank greater than m — 1.
4. PX contains only finitely many points of rank greater than m — 1.
5. C{/3X) is mquasinormal.
108
Proof: The implications (2) =^ (3) =» (4) =^ (5) have the exact same proofs as in
[La2, 4.3], using Lemma 5.3.1.
(5) =^ (2) : Let {co2;(/j)}J^i be a pairwise disjoint set of cozerosets of X, and for
each i, let /f be an extension of f, to PX so that {coz{fi)}iLi is a pairwise disjoint
family. Then fllli dfix{coz{ff)) is finite by Theorem 5.2.3. Since fli^i dx{coz{fi))
is contained in this finite set, it too is finite. Thus, (2) holds by Theorem 5.2.3.
(1) =j> (5) : Follows from Theorem 5.1.5.
(3) ^ (1) : Since (3) implies (5), we have that C[X) is 77iquasinormal. Condition
(3) also implies that xk^xip) = 1 for all p € f5X \ X, by Lemma 5.3.1. ■
Recall that an /ring A is called an SVring if A/P is a valuation ring for
every prime ideal P. The following arises when in pursuit of conditions which imply
that C{X) is both r/vquasinormal and SV.
Theorem 5.3.3. LetX be normal andm > 2. IfC{X) is mquasinormal and every
maximal £ ideal has finite rank, then X satisfies the {Qmte,m) boundary condition.
Proof: Assune that C{X) is mquasinormal, that every maximal £ideal has
finite rank and that {Uj : I < j < m} is a family of pairwise disjoint cozerosets
such that the set W = rij^idxiUj) is infinite. Then there exists a copy of N in W.
Denote this copy by F = {xj : j € N}. Then Y is not closed in PX, so there exists
p € /3X such that p e d^xiy) \ Y Let {t/j : 1 < j < m} be cozerosets of PX such
that U'jDX = Uj for each j. Then p ^ C/j for all j = 1, 2, ..., m and p e nf^id^xU'y
For each i, Xi E W and hence rkx{xi) > m. Let Pii,P2t>"jfmt be distinct
minimal prime ideals of C{X) such that we have Pji C M^^ for all j = 1,2, ...,m
and Uj ^ Uife/jC02;(Fjfci). Let p 6 PX and rkc{x)iM^) = n < oo. Then as shown in
the proof of [L,4.2], there exists a minimal prime ideal Q C Af such that for every
Z e Z{Q) such that Z n F is infinite.
109
For every / G C{X) define JJ = {xi G Y : f e Pji} for j = 1, 2, ..., m. Let
^ be the 2ultrafilter on Y containing the zfilter {Z HY : Z e Z{Q)}. Define for
j = 1,2, ...,m, i2j = {f e C{X) : Jj e J^}. As shown in [L,4.2], each Rj is a prime
ideal of C{X).
First we show that the Rj are noncomparable ideals. For each Xj G F, let C/j
be a neighborhood of Xj such that the collection of these neighborhoods is pairwise
disjoint. For each i e N and for each 1 < /, fc < m, let fiki e C{X) such that
fiki e Pii \ Pki, f{X \ Ui) = 0, and < /,« < ^. Then let /,* = YlZi hki FinaUy,
fik^RiX Rk and /« eRk\Ri for all 1 < i, A; < m, / ^ A:.
Let /2j C /2j be a minimal prime ideal for each j = 1, 2, ..., m. Larson demon
strates that Rj C MP for all j. We will show that Yl'JLi ^j is neither the maximal
£ideal Mp nor all of C{X).
Define h e C{dxY) by h{xi) = 4 for i = 1, 2, ... and let h{x) = otherwise.
Then since X is normal and dxY is closed, h extends to a function h eC{X) such
that h € M". We show that h i Xl^i ^i For each j = 1, 2, ..., m, let /, e ilj. Then
jj. 6 :F and hence f%iJ}. G :F and (X;7=i /j)(xi) = for every Xi G nf^^ J}.. If
h = 5]^i /j then since h{xi) / for all Xj G F, = n^^ Jj. G :F, a contradiction.
Thus /i G M" \ X:7^i ^i and therefore, E^^i ^j ^ Er=i Rj S ^^ ^^ ^(^) ^^
not T7>quasinormal, a contradiction. Thus for any m pairwise disjoint cozerosets
{Uj : 1 < j < m}, the set Hf^^dxUj is finite. ■
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BIOGRAPHICAL SKETCH
Chawne Monique Kimber was bom in Frankfort, Kentucky, on January 12,
1971. She was raised in Tallahassee, Florida, where she graduated from Leon High
School in 1988. Chawne received the Bachelor of Science in Mathematics from the
University of Florida in 1992, and the Master of Science in Mathematics from the
University of North Carolina at Chapel Hill in 1995.
113
I certify that I have read this study and that in ray opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of philosophy ^^
" limk
Jorge Martinez , Chairman
Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
ichard Crew
Associate Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
Alexander Drahishnikov
Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
Scott McCuUough
Associate Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable stajidards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
Mildred HillLubin
Associate Professor of English
This dissertation was submitted to the Graduate Faculty of the Depaulment
of Mathematics in the College of Liberal Arts and Sciences and to the Graduate
School and was accepted as partial fulfillment of the requirements for the degree of
Doctor of Philosophy.
May 1999
Dean, Graduate School
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